Suspected gang members torched three public buses and fired shots at police, sparking an alert by authorities fearing the violence could mushroom into a repeat of a huge crime wave last year, officials said Wednesday.

No one was hurt in the attacks Tuesday night, but authorities were investigating whether they were orchestrated by the First Capital Command gang, whose campaign of assaults on police, banks and buses last year left more than 200 dead in South American's largest city.

"Police are on an above-normal level of alert," Public Security Secretary Ronaldo Augusto Marzagao told reporters. "But there's no reason for people to get alarmed."

In the bus burnings, men on motorcycles stopped the vehicles, ordered passengers and drivers off and then doused the buses with gasoline and set them ablaze.

Nearly simultaneously, suspected gang members on a highway overpass opened fire with submachine guns on a police patrol car parked below them on the street. The officers were away from the cruiser at the time.

Four armed men were arrested early Wednesday on suspicion of participating in the attacks, said a spokeswoman for Sao Paulo's public safety department who would not give her name because of department policy.

Globo TV also broadcast footage showing three police officers beating suspects rounded up in a drug sweep early Wednesday in a dangerous Sao Paulo neighborhood, as the suspects were lined up against a wall. The officers then burned some of the suspects' belongings, the network reported.

Successive waves of gang-initiated violence in Sao Paulo and Rio de Janeiro have shocked even crime-weary Brazilians, and led to allegations that Sao Paulo officers exacted revenge in poor neighborhoods by executing innocent people while cracking down on gang members.

Sao Paulo state police killed 533 alleged criminals last year in incidents officially registered as "resistance followed by death" — nearly 80 percent more than the 300 estimated to have been killed in gunbattles in 2005.

Over the weekend in Rio, paramilitary militias and drug traffickers engaged in gunbattles that killed three suspected gang members and two police officers and injured at least 10 people. In January, a series of attacks by suspected gang members on police and civilians around the city killed 19 people.

Last month, heavily armed gunmen ambushed and killed a prison director in Sao Paulo, and police discovered a plan allegedly linked to the First Capital Command to kill other prison directors throughout the state.

President Luiz Inacio Lula da Silva announced this week that Brazil will spend US$183 million (euro140 million) to ensure safety in Rio during the Pan American Games in July. About 500 elite federal officers were sent to the city last month, and as many as 5,000 could be on patrol when the Olympic-style games begin.

The attacks last year in Sao Paulo brought commerce to a standstill periodically. But Tuesday's violence appeared to have little impact on daily life in the sprawling metropolis of about 18 million.