Something is seriously wrong, and a crowd of people — usually common, lower-class, etc. — has gathered. They are discontented and angry, but not really whipped up. They don't really have a plan in mind.
This can last a long, long, long time, with no overt effect except some hostile looks, but eventually something will happen.
Someone provokes them, possibly from more-or-less legitimate fear of how bad it could get. Particularly likely when a Blue Blood doesn't take them seriously. Then the mob really is whipped up. Lots of damage all around. Although most of the dead are likely to be in the crowd, it really does get that bad. (Torches and Pitchforks is not likely; the mob is not focused and will likely destroy whatever they get their hands on, which means they are often far more destructive than Torches and Pitchforks.) If the crowd has weapons, it may become a Blast Out or a Molotov Cocktail throw-fest.
In the Wild Series, every time the manbeasts go into human settlements, the humans start getting suspicious and violent because they subliminally pick up the destructive power the manbeasts are capable of. This in turn makes the manbeasts lose their control and involuntarily react, escalating the situation.
It's not exactly easy to animate a riot scene. Unless of course, computers take away the drudgery. They pulled it off with Metropolis and one of the Animatrix shorts.
In Watchmen, the whole prison goes berserk when an inmate finally dies from injuries inflicted by Rorschach.
Showed up in Diabolik in the story "The Great Escape", where the volatile political situation in Benglait explodes in a brief civil war when crown prince Danilo has the royal guard fire on protesters.
Toward the end of Do the Right Thing, Mookie intentionally turns an Angry Mob into a full-blown riot by picking up a trash can and throwing it through a diner window (the owner having been very distantly connected to the death of one of his friends).
Also toward the end of Hot Rod, a crowd of people are following the protagonist through the streets, to the sound of 80's power rock music. Everything's great, people are singing, everyone's getting hyped up for The Big Jump. When, all of a sudden, someone throws a trashcan through a window. Riot police appear of nowhere, and eventually, it's a full-scale riot.
Carefully explained by the detective in V for Vendetta as the little girl with the broken glasses is shot, leading to the first type of result. Then the same thing is set up with the crowd at the houses of parliament.
This seems to be a common theme in Soylent Green, especially in the marketplace when masses of people (disgruntled over a shortage of Soylent Green) break out into a riot.
In the Book Of Eli post-apocalyptic world, this seems to happen in the town bar/pub on a regular basis.
Illustrated very well in the 2012 film version of Les Miserables. A crowd has gathered for the funeral of a popular military figure, General Lamarque. The atmosphere is charged, more so when a group of young revolutionaries start up an anthem and hijack the funeral cortege. Suddenly a soldier fires on a bystander. Uproar ensues.
In The Hunger Games, a crowd in District 11 is shown watching the Hunger Games. When Rue dies and Katniss salutes her, an angry man in the crowd starts attacking the guards, provoking a full scale riot.
Nathanael West's The Day of the Locust climaxes with a Hollywood movie premiere at which the crowd of star-gawkers gets more and more restless and agitated, and finally explodes into a full-scale riot after a young child actor gets stomped to death.
It's disturbing enough in the book, but John Schlesinger's movie version takes it to the level of full-on Nightmare Fuel.
The provocative case occurs off stage in Terry Pratchett's Night Watch and results in many deaths. The defused kind turns up when Vimes expends a great deal of effort ensuring that it doesn't happen at his post.
Another instance occurs in Guards! Guards! when an angry crowd surrounds Lady Sybil's dragon sanctuary; on this occasion Vimes manages to calm them down.
It should be noted that it's been repeatedly said that Ankh-Morpork exists in a perpetual state of "proto-mob".
In Unseen Academicals, the Shove is always a Powder Keg Crowd. Neatly defused by Nutt at the climax.
In Thud!, trolls and dwarfs. Even, alas, in the Watch. At one point, Vimes notices how rumor is spreading through a crowd of dwarfs, and thinks "This crowd was waiting uncertainly for news that it was going to become a riot".
In John Barnes' One for the Morning Glory, Prince Amadeus faces such a crowd and manages to persuade them that they had come to draw matters to his attention, and that he would deal with them.
In Stephen King's Wizard and Glass, Roland recounts his backstory where he lost his One True Love. He lost her due to a Powder Keg Crowd whipped into a killing frenzy by Rhea of the Coos. Said crowd literally burned Roland's love alive.
In Graham McNeill's Warhammer 40,000Ultramarines novel Nightbringer, when a crowd is pressing in on the governor, a sergeant orders the Arbites to open fire, over his captain's orders, killing many, until the captain manages to override them. Later, a demonstration is deliberately fired on by men disguised as Arbites, and the resulting riot kills thousands and spreads destruction about the city.
In A Clash of Kings, King Joffrey and the other Lannisters are riding through a resentful crowd of starving peasants on their way back to the castle. Someone throws dung at Joffrey. Furious, Joffrey orders the Hound to cut his way through the crowd and retrieve the offender. This triggers a riot that spreads throughout the city.
In Dan Abnett's Gaunt's Ghosts novel Necropolis, Vervunhive Commissars attempt to stop refugees from going into sealed off areas. When throwing about "State of Emergency" doesn't work, they shoot several to scare them off. Not a good idea when the odds are six armed troopers facing several hundred unarmed refugees.
The main character in Invisible Man has a knack for manipulating these crowds towards what he hopes will be the best possible course of action. Ras, the resident Dark Messiah, just gets them to destroy things.
The Rifter: In Gisa, when the prisoners accused of witchcraft are due to be taken to the Payshmura temple by a troop of rashan’im soldiers, a huge crowd gathers in front of the prison gates, singing and shouting to the soldiers to go home. The guards at the gate don’t interfere. Even the soldiers hesitate, but the situation is hanging on a knife-edge between them leaving or starting a massacre, when The Cavalry arrives in the form of kahlirash’im warrior-priests who turn out to not agree with the Payshmura priests at all. The rashan’im turn tail.
In Stephen Hunt's The Court of the Air, when a crowd gathers about the castle, the figurehead/scapegoat king comes out on the balcony to let them throw fruit and rubbish at him, sating their desire to riot.
At the end of 1633, one of these was gathering in Magdeburg after word of the death of Hans Richter reached the general public. Mike Stearns and company defuse the situation before it actually blows up, though.
Examined in 1635: The Cannon Law, where it is made blindingly obvious that the crowd is only there because somebody who thinks he's The Chessmaster paid them to do so. Several of them aren't even sure what they're gathering around angrily protesting, just that they're getting a nice bit of gold to stand in the square and yell incoherently.
In 1635: The Dreeson Incident, what was supposed to be a protest against vaccination at a Grantville hospital turns into a protest against autopsies (a much bigger religious issue at the time). When a policeman takes offense at the insults being thrown, he draws his gun and fires, killing a protester. The policeman is then killed by the furious crowd and a major riot erupts with multiple deaths on both sides.
In Tamora Pierce's book Bloodhound the combination of rampant counterfeiting of silver, and a potential blight in a major grain crop turn the entire capital into this with a massive riot starting after a shop owner doubles the price for bread in the slum district.
In Jack Campbell's Lost Stars novel Tarnished Knight, the loss of the security forces brought about this. Drakon acts quickly to contain it, publicly reminding the population that they weren't out of the woods yet and people urging violence and destruction may be ISS agents, and setting guards on crucial locations.
In Sandy Mitchell's Ciaphas Cain novel "For the Emperor", the regiment. It does erupt in a murderous riot, and coping with it puts him to the test. After he's dealt with that, he then has to deal with a much larger version of this when the regiment's assigned to a whole planet which is in this state thanks to the Tao's attempts to annex it (in their usual Bread and Circuses fashion). It quickly turns out that someone is trying to set off the civil war. It turns out to be a Genestealer cult trying to sew disorder before a Tyranid invasion.
The striking mill-workers in North and South, who angrily swarm up to employer John Thornton's house and very nearly start to riot.
Halo 3: ODST has a crowd trying to get on a subway train that the corrupt police commissioner has commandeered. His men are shooting anyone that gets too close, and when it seems like he's about to accomplish what he's been trying to do for the entire audio story (rape the girl that the story centers around) her companion manages to get the crowd riled up enough to overwhelm the cops and break onto the train, ripping the guy to shreds. Literally.
In Suikoden V, the Prince finds out that Salum Barrows took advantage of one of these to steal the Dawn Rune
In Deus Ex: Human Revolution, these crowds keep showing up: in the prologue, there's one waiting in Washington D.C. for the Senate hearing that Adam and Megan are heading to, and there was another one just outside the LIMB clinic in Detroit in the first act when Adam finally gets to the streets. A third crowd gathers outside Sarif Industries in the third act, and almost immediately explodes when the Illuminati leak "evidence" of terrifyingly unethical augmentation experimentation. Fully a quarter of Detroit is inaccessible as a result of the ongoing riot, and the rest of the area has massive police presence.
In the SpongeBob SquarePants episode "Bubble Buddy", a crowd of people become enraged at the eponymous bubble. Once they're set off, they tip over the the lifeguard stand and attempt to kill the lifeguard before focusing their rage on Bubble Buddy.
Recess. The students at Third Street Elementary are astonishingly easy to provoke.
According to The Boondocks, all it takes to turn a mildly disgruntled crowd - especially a Black crowd - into a full blown riot is someone throwing a chair. This starts off sounding like Riley's normal level of hyperbole, until he actually does so: the crowd stops, one guy yells "I'M MAD" and punches someone else at random, and it all instantly descends into an orgy of violence. This, despite the fact that it was Riley the crowd was angry at, and that the chair didn't even hit anyone.
Famously the Romanian revolution against Nicolae Ceauşescu. In a time of great public unrest it probably wasn't the smartest idea to round up "supporters" to attend a rally to promote loyalty to the regime. A few minutes into the presidents speech, the crowd got fed up with his delusional talk and everything went downhill within minutes, forcing Ceauşescu to retreat inside the building. To make things worse, the whole event was broadcast live on TV. The next morning the military joined the protesters and three days later Ceauşescu was executed.
During 1848, there was a string of nationalist revolutions, revolutionary attempts and riots all across Europe. England avoided violence. Historians suggest this is because the English police weren't given firearms when they were sent to overlook protests.
The 1863 New York City draft riots. People were annoyed at the draft (which allowed the wealthy to buy their way out) and the Emancipation Proclamation (many whites not thinking that abolitionism was a cause to die for, and many of the working class being pro-slavery). What was the spark? A fireman was drafted. Three days later, the riot ended. The total dead is still debated, with estimates ranging from 120 to 2,000.
On a similar note, the two Arab revolutions in 2011 that fully succeeded—Egypt and Tunisia—were ones in which the regime fired upon and otherwise brutalized the protesters (who were almost entirely if not entirely peaceful). Not for nothing did one Egyptian journalist write an article titled "Thank you for shooting at us!" The three protest movements that kind of just fizzled out—Algeria, Morocco, and Jordan—were the ones where the police did not fire into the crowd; the Moroccan police were specifically given strict orders to give the protesters a wide berth and not to fire.
Most analysts agree that the decision to fire on peaceful, unarmed civilians attracted sympathy for the protesters that would not otherwise have existed, which in turn led to increased turnout at the protests. This is borne out by the results in Libya, Yemen, and Syria, where the protest movement grew each time it was reported the police or army had fired upon civilians; this led Syria and Yemen into a state of vague turmoil, and led the Libyans to take up arms. The only way (it seems) for a strategy of brutal violence to work is if it is done whole-hog and very quickly; the Bahraini government managed to crush the protest movement, but only because hardliners did not hesitate to use the military to break up the sit-in at Pearl Roundabout and then call in Saudi and Emirati troops to quell the uprising once and for all.
Also, a government which won't fire on its citizens is a good candidate for genuine reform, while a regime which kills its own people kind of needs overthrowing.
Though this was, notoriously, inverted by the Tiananmen Square protests in China; twenty years later, there has still been no reform worth speaking of, let alone anything like overthrow.
The day after MLK Jr. was shot James Brown performed at the Boston Garden. Mayor Kevin White arranged to have the concert broadcast on local public television, and there was no rioting in Boston. Brown was thus given credit for saving Boston from burning.
In Chicago, the same day, the situation was not defused and rioting broke out. Three days later it was finally over, with 11 dead, over 500 injured and 3,000 arrested. Over 200 buildings were destroyed and nearly 1,000 were left homeless by fires.
There were Powder Keg Crowds in many major US cities leading up the Rodney King verdict, and there was some rioting in all of them. The most violent aside from L.A. were across the South, from Dallas to Atlanta.
During Mao's Cultural Revolution in China, people were instructed to hunt down anyone suspected of being an "enemy of the people." Many of the victims were teachers, artist, doctors, religious followers. Those who were caught by Mao's Red Guard were dragged into the streets and publicly vilified by a jeering crowd. This was mandatory, and anyone not attending to these were considered enemies themselves.