The fickle crowd of Roman plebians that gather after the assasination of Julius Caesar is influenced by the words of both Brutus and Marc Antony. When the citizens first hear the reasons that Brutus propounds for the slaying of Caesar, that he loves Rome more than he does Caesar, and that he and the others slayed Caesar to save Rome from tyranny and keep them free, they cheer Brutus and tell him to live, rather than sacrifice his life at their demand. In fact, some want to erect a statue of him, while others suggest that he become a Caesar--"let him be Caesar"; moreover, one citizen even says,

Caesar's better parts
Shall be crown'd in Brutus. (2.2.53-54)

However, after this same crowd listens to Marc Antony's oration, they are easily moved by his rhetoric; consequently, they begin to doubt Brutus and the others' noble deed in slaying Caesar. For, Antony has raised suspicion in the crowd regarding the motives of the conspirators by saying such things as

You all did love him once, not without cause;
What cause withholds you then to mourn for him? (3.2.110-111) 

As he kneels over the body of Ceasar that he tells the crowd has been disfigured by traitors, Antony directs the crowd to the many and brutal wounds of Caesar,

Look, in this place ran Cassius' dagger through;
See what a rent the envious Casca made;
Through this the well-beloved Brutus stabb'd;
And as he pluck'd his cursed steel away,
Mark how the blood of Caesar follow'd it, (3.2.184-188)

Further, Antony stirs the Romans suggesting that Brutus and the others want their inheritances from Caesar; moreover, Antony says if he were Brutus and Brutus Antony, this Antony

Would ruffle up your spirits and put a tongue
In every wound of Caesar that should move
The stones of Rome to rise and mutiny.(3.2.238-240)

With his words, Marc Antony spurs the Roman crowd to riot, and they run to set fires, tear down benches, break windows, and create civil unrest.  Most significantly, the civil war that the Roman citizens begin causes the separation of powers with Brutus and Cassius against the Triumvirate of Marc Antony, Caesar Octavius, and Marcus Aerilius Lepidus; after Brutus and Cassius's forces are defeated, the provinces are divided into spheres of influence.