Caesar approaches the Capitol with the conspirators, followed by Antony, Lepidus, Publius, Popillius, and other Senators. Caesar notices the Soothsayer, and tells him his prophecy hasn't come true; the Soothsayer says the day's not over yet. Artemidorus tries to hand Caesar his letter, but is blocked by Decius and Cassius. Popillus wishes Cassius good luck, and Cassius realizes that word of their plans is spreading, which means they must be quick.
The Soothsayer's final line is more like cryptic gloating than a warning. There are now others besides Artemidorus who know of the conspiracy, but are sympathetic to it. It's never made clear whether one of the conspirators leaked information, or if they were spied on.
In the Capitol, Trebonius talks with Antony, to draw him away. Metellus kneels before Caesar to beg for the repeal of his brother's banishment. The other conspirators join him, in order to position themselves near Caesar. Caesar refuses to reverse his order, comparing himself to the North Star in terms of constancy: "there's but one in all that holds his place" (3.1.65)
Caesar's egotistical boasts about his constancy make him less likable (which makes Brutus more likable to the audience), but also calls Brutus's logic into question: Caesar seems determined not to change, and Brutus's decision is based on believing that he will.
The conspirators stab Caesar—Casca first, Brutus last. Caesar's last words are "Et tu, Bruté?—Then fall Caesar" (3.1.76). The conspirators attempt to start a rallying cry about Liberty, but a panic ensues and many Senators flee. Brutus sends Publius to tell the other Senators that no harm will befall anyone else. On Brutus's advice, the conspirators smear themselves with Caesar's blood, and prepare to march forth into the streets.
Caesar supposedly stopped defending himself when he saw that Brutus was one of his attackers. Their bathing in Caesar's blood confirms Calpurnia's dream, but their slogan about liberty seems ironically unpersuasive: they have "delivered" Rome from things that haven't happened yet.
Antony's servant enters with a message. Antony sends word that he will support Brutus if he may safely approach and be given a satisfactory explanation for Caesar's death. Brutus praises Antony and grants the request. Cassius remarks that he still doesn't think Antony can be trusted.
It is unclear whether any explanation could pacify the passionate Antony, but the logical Brutus seems convinced that his will. The contrast between Brutus's rhetoric and Antony's begins here.
Antony enters, and is moved by the sight of Caesar's body. He says that if the conspirators intend to kill him, they should do it now, as seeing Caesar dead has made him ready to die. Brutus and Cassius tell Antony that they mean him no harm, and that he'll have an equal voice in the establishment of a new government. Antony shakes hands with them, and apologizes to Caesar's spirit for doing so. He asks permission to speak at Caesar's funeral, which Brutus grants, despite Cassius's objections. They agree that Antony should be the one to bear Caesar's body outside, and all but Antony exit.
Brutus does not suspect that Antony means to use his speech at Caesar's funeral to turn the people against the conspirators because the idea of emotion overpowering logic or honor is foreign to him. Cassius, who had always been ruled by emotion (his jealousy) does suspect Antony,.
Alone, Antony predicts that a terrible war will engulf Rome as a result of Caesar's murder. A servant of Octavius enters to tell Antony that Octavius has almost arrived in Rome. Antony sends word that it's too dangerous for Octavius to enter the city, and that their actions must depend on how the citizens react to Caesar's death.
Octavius is Caesar's heir, but their cause now rests on Antony. Rome is, for the moment, without a government, and Antony correctly observes that power will go to whoever can win the hearts of the people.