Gangs torched buses and attacked banks and police stations across South America's biggest city before dawn on 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. And while banks, shops and restaurants were open for business on Thursday, the violence was much on the minds of people downtown.

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"It's getting too scary," said 30-year-old Alexsandro Pereira, a bank employee in the financial center. "What's even worse is that we don't see a solution to this. The politicians keep fighting each other and we don't know what's going to happen to us."

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 state of Sao Paulo. Nearly 200 police, prison guards, suspected criminals, and jail inmates, along with some allegedly innocent bystanders, died 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 on Thursday, as well as his leading challenger, Geraldo Alckmin, who governed Sao Paulo state before announcing his bid for the October election.

Silva has 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."

Sao Paulo State Gov. Claudio Lembo, a close ally of Alckmin, said federal help wasn't yet necessary, but was expected to discuss the tenuous security situation with Justice Minister Marcio Thomaz Bastos later Thursday.

The crime violence could potentially harm Alckmin and his Social Democratic party more than Silva, who would probably win re-election in the first round of voting if balloting were held today, according to a poll released Tuesday.

Silva, of the leftist Workers Party, would get 44 percent to Alckmin's 27 percent, according to the Sensus poll, which had an error margin of 3 percentage points. But the survey showed growing support for Alckmin, who had 20 percent in the previous Sensus poll.

"It is easier for citizens to blame the situation on Alckmin. After all, these waves of violence are taking place only in Sao Paulo, Alckmin's political home. The rest of Brazil is dealing with their own regular problems, but not with criminal waves," said Alexandre Barros, a Brasilia-based analyst.

Seven people were killed on Tuesday and Wednesday, including a police officer, a civilian and three private security guards, Sao Paulo's public safety department said. Media reports 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 of the 15,000 buses, critical in a city of 18 million, including metropolitan areas, were kept off the streets. Police agents in plainclothes were inside some transport units to help prevent attacks.

Tens of thousands of commuters were stranded in bus stations. Many others, specially those living in poor neighborhoods, remained at home.

Authorities said automatic rifles, pistols and firebombs were used in the attacks. Local media reported a homemade bomb exploded overnight in the city hall of Juquitiba, about 45 miles south of Sao Paulo, and that an ambulance was torched after gang members made a fake emergency call.

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.