SAO PAULO, Brazil (AP) -- Gangs torched buses and attacked banks and police stations across South America's biggest city Thursday, deepening crime fears as a wave of rampant violence entered its third day.

At least six people have been killed in nearly 100 attacks on police and civilian targets since Tuesday night in what authorities believe is retaliation for a possible transfer of gang leaders to Brazil's first federal penitentiary.

Sao Paulo residents had trouble getting to work Thursday morning after several public transportation companies refused to dispatch buses out of fear of new attacks.

The violence comes two months after imprisoned leaders of the gang First Capital Command -- known by its Portuguese initials PCC -- 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 in one week.

The new wave comes at a critical time for President Luiz Inacio Lula da Silva -- who was officially launching his re-election campaign Thursday -- as well as his top challenger Geraldo Alckmin, who was governor of Sao Paulo state before announcing his bid for the October election.

Silva on Wednesday offered to send in a federal security force, saying "the situation is grave because we are not dealing with common criminals, it's a crime industry." But Sao Paulo State Gov. Claudio Lembo said federal help was not yet necessary.

Lembo was expected to meet with Justice Minister Marcio Thomaz Bastos on Thursday to discuss the security situation.

Six people were killed Tuesday and Wednesday, including a police officer, a civilian and three private security guards, Sao Paulo's public safety department said. Some media said at least nine have been killed in the attacks, but those reports were not immediately confirmed.

A 2-year-old boy and his mother remained hospitalized after assailants torched a bus in the coastal city of Santos, some 50 miles southeast of Sao Paulo.

More than 50 buses have been set on fire since Tuesday, authorities said. One of the buses torched Thursday was in a middle-class Sao Paulo neighborhood known for trendy bars and restaurants, which were forced to close.

The city's subway system was working but more crowded than usual as most public transportation companies kept their buses off the streets.

Authorities said automatic rifles, pistols and firebombs were among the weapons used in the attacks Thursday. Local media reported a homemade bomb exploded overnight in the city hall of Juquitiba, about 45 miles south of Sao Paulo.

An ambulance also was torched after gang members allegedly made a fake emergency call, local media reported.

Seven people have been arrested since Tuesday, including Emivaldo Silva Santos, believed to be a leader of the PCC.

Public Safety Secretary Saulo de Castro Abreu said the latest attacks were sparked by reports of an imminent transfer of gang leaders from Sao Paulo prisons to Brazil's first federal penitentiary in the southern state of Parana.

The PCC was originally formed in the 1990s to press for improved prison conditions, but it quickly began using its power inside prisons to direct drug and arms trafficking, bank holdups, kidnappings, extortion and killings on the outside.

The PCC is believed to exert influence over 95 percent of the 140,000 prison inmates in Sao Paulo state.

Another reason I want all the illegal brazillians out of city!