Commuters had a tough time on Thursday morning after several public transport companies refused to run buses after more than 50 were torched since Tuesday.
Six people were killed on Tuesday and Wednesday, including a police officer, a civilian and security guards, Sao Paulo's public safety department said.
A 2-year-old boy and his mother remained hospitalized after assailants torched a bus in the coastal city of Santos, some 80km (50 miles) southeast of Sao Paulo.
Almost 100 attacks have been carried out on buses, police stations, courthouses, banks, supermarkets, car dealerships and other establishments since Tuesday night, allegedly in response to reports that gang leaders might be moved to a remote federal penitentiary.
Seven people have been arrested, including Emivaldo Silva Santos, believed to be a leader of the First Capital Command gang - known by its Portuguese initials PCC.
Sao Paulo's state police commander hinted that the recent attacks were ordered by the group, saying they are the work of "criminal organizations" - a phrase authorities often use to refer to the PCC.
The violence comes two months after imprisoned leaders of the PCC allegedly ordered attacks against police across the city and Sao Paulo state, causing the deaths of nearly 200 police, prison guards, suspected criminals and jail inmates within a week.