In Act I, Scene 1, the tribunes, Flavius and Marullus, are offended by the fickle nature of the commoners. They first chastise the cobbler and the carpenter because they are not at work, but when they learn that these men are celebrating the death of Pompey's sons, whom Caesar has just defeated, they are incensed:
Have you not made an universal shout/That Tiber trembled underneath her banks/To hear the replication of your sounds/Made in her concave shores?.../And do you now strew flowers in his way/That comes in triumph over Pompey's blood? (I,i,45-53)
In this exposition to Shakespeare's play, the tribunes point to the capricious nature of the Roman crowd that can be so easily swayed from one side to that of another. For, once they cheered Pompey as a great general. Now, they cheer the death of "Pompey's blood?" (Pompey's sons). Indeed, this scene foreshadows the facile persuasion of the crowd against Brutus by Marc Antony in Act III.
In Julius Caesar, the carpenter and the cobbler take a day off of work to celebrate Caesar's entrance into Rome following his victory in a civil war against another Roman general. As the Cobbler tells Flavius and Marullus:
make holiday to see Caesar and to rejoice in his
triumph. (Act 1.1.33-35)
Flavius and Marullus are upset by this, they say, because Caesar has only defeated another Roman, not conquered an enemy. They want to know who Caesar brought back to Rome who will pay monetary tribute to the city after having been conquered. The implied answer is no one, of course, since it was only a civil war. The two chase away the commoners and "disrobe the images" (line 69), or take off the ceremonial scarves honoring Caesar from Rome's statues.
I'll let another editor handle what happens to Flavius and Marullus because of their actions, if that's what you're asking. There are no slaves in the opening scene, though. The cobbler and the others are commoners, but they're not slaves, and Caesar doesn't do anything to them. Flavius and Marullus are the ones who are punished.
I believe that you are talking about what happens at the beginning of the play -- right at the start of Act I, Scene 1.
The first thing we see is the tribunes Flavius and Murellus scolding a carpenter and a cobbler. The reason they are scolding the two men is that the two men are not working. Instead, they are out taking a holiday.
The reason that the men are taking a holiday is because Julius Caesar has declared one. He is holding a triumph -- a parade in celebration of a victory. This offends the tribunes because Caesar has won a civil war and not a war against foreigners.