Historians often comment on mob rule as a factor in the rise of Rome, and its maintenance, as the city of Rome itself was huge (reaching a million people in ancient times, unheard of), and the aristocracy and even military very small by comparison to the citizenry. Weapons also being crude, there was a constant need to keep the mobs fed, distracted, and in awe of the power of the state. Who could do this, could rule not just Rome, but the whole Roman Empire.
Lapses in this control often led to loss of power, or even the loss of heads, of officials - most notably in the reign of Commodus when Cleander unwisely used the Praetorian Guard against a mob which had come to call for his head. As Edward Gibbon relates it, "The people... demanded with angry clamors the head of the public enemy. Cleander, who commanded the Praetorian Guards, ordered a body of cavalry to sally forth and disperse the seditious multitude. The multitude fled with precipitation towards the city; several were slain, and many more were trampled to death; but when the cavalry entered the streets their pursuit was checked by a shower of stones and darts from the roofs and windows of the houses. The foot guards, who had long been jealous of the prerogatives and insolence of the Praetorian cavalry, embraced the party of the people. The tumult became a regular engagement and threatened a general massacre. The Praetorians at length gave way, oppressed with numbers; and the tide of popular fury returned with redoubled violence against the gates of the palace, where Commodus lay dissolved in luxury and alone unconcious of the civil war... Commodus started from his dream of pleasure and commanded that the head of Cleander should be thrown out to the people. The desired spectacle instantly appeased the tumult;"
This followed a previous incident in which the legions of Britain had demanded and received the death of Perennis, the prior administrator. The mob thus realized that it had every chance of success. This is a typical feature of mob rule situations - emboldened by success, a mob demands ever more power and sanctions.
During the French Revolution, the mobs in Paris played a similar function, but were more carefully manipulated by political leaders who sensed that they had the power to dispose of monarchy entirely, as they did, eventually setting up a representative democracy (which in turn fell to Napoleon's model of semi-constitutional monarchy).
The modern theories of civil disobedience and satyagraha have taken something from mob rule and its mechanics. Certainly it is quite frightening for large numbers of people, especially peaceful ones, to be marching and shouting common demands, if one is charged with the uncomfortable task of refusing them. If Roman guards, facing crucifixion for disobedience, could be swayed by mobs, it is obviously possible also to sway modern police even in a police state. The Velvet Revolution in former Czechoslovakia, and the resistance to the military coup in the Soviet Union in 1991 that led to the collapse of that state, were situations where the "mob" won the day due to defections by authority.
A situation where they did not win was Tiananmen Square in 1989. While initially Beijing-based units of the People's Liberation Army refused to charge on the students occupying the square, new units from the countryside were brought in who tended to perceive the students not as citizens like themselves but as spoiled people with every privelege. Slaughter followed.
As this example shows, relying on sheer mob strength and disruption is chancey in any political movement, and is not to be relied upon for any extended period. Anarchism is a theory of civics that relies on mobs and passive resistance more than most, but all branches of it stress the need for ethical relationships and voluntary association. This is not a good description of a "mob", which generally lacks anything that one might actually describe as real "integrity" or an explicit "ethical code". Encounters with mob rule usually hinge on threats of bodily harm - do what the mob wants, and you won't get hurt, resist, and you almost certainly will - the sheer size of the mob making it difficult or impossible to assign blame to any one person.
The term "mob" is also sometimes used to describe organized crime. Since it is relatively simple for the criminal element to exploit public strife, e.g. by looting, or grabbing power by means of fraud in the confused circumstances, there is some resonance in the idea of "mob rule" meaning, ultimately, rule by these people who exploit or create mobs by leading them into violence.
A common observation of the long history of representative democracy is that, very often, it is physical control of polling stations that determine who wins: whoever can bring out more supporters (typically with clubs and farm implements, although not usually knives and guns) to keep the opposing political party out, wins. Political privacy is very often nonexistent in this kind of condition, so retribution against defectors is easy. This may simply represent a transition from simple mob rule with an aristocracy in charge of organizing rival mobs - this being difficult to tell from the organized crime exploitation scenario.
Some critics view the anti-globalization movement's protest against G8 and World Trade Association and IMF meetings as an attempt to impose mob rule. This may be valid, as such groups have actually managed to change at least part of the agenda, and timing, of such meetings, and forced leaders to address their concerns, out of proportion, some say, to the degree to which they are shared in the populace.
See also: tribalism