Generally enslavement of Roman citizens didn't happen--it simply didn't make sense, when both sides are appealing to the citizen body to support them, apart from the fact that enslavement of Roman citizens was on the list of things so horrible that no Roman would ever do it, lest he be eternally dammed, which was not something any leader wanted. However, that doesn't mean it wasn't feared--Cicero likes to harp on how Catiline wanted to enslave the citizens, which seems to be purely rhetorical--and it doesn't mean that nasty things didn't happen to Roman citizens. During the several rounds of civil war there were very few sieges, but while most open fighting occurred outside Italy, most of the fighting in Italy consisted of mass raids and extremely destructive sieges. Some nasty things happened during the Social War, but those people were technically not citizens generally. Proscriptions were quite common--Augustus in later years tried to cover up the fact that he had been responsible for the biggest round of proscriptions, murdering or exiling thousands of his political enemies. The closest thing we have to what you're asking for is the massacre of the senate of Perusia during the assault on the city. Supposedly Octavian ordered about 200 of the members of the Perusine assembly to be massacred, but it's not supported by all of our sources and it's somewhat doubtful whether it ever occurred. It generally wasn't cities that suffered--it was the countryside. The wars between Sulla and Marius famously displaced thousands of farmers, flooding the city with urban poor who were not accustomed to life in the city, which drove several campaigns by certain politicians to impose land reforms.