Police in South America's largest city have been placed on maximum alert following a shootout in which 13 suspected gang members were killed trying to carry out a plan to attack prison guards.

All 13 were presumed members the First Capital Command gang that in mid-May unleashed an unprecedented weeklong spree of violence that left nearly 200 people dead.

The gang, better known by its Portuguese initials PCC, is one of Brazil's most notorious organized crime groups.

Twelve men and one woman were killed Monday in a gunfight with police outside a prison in Sao Bernardo do Campo, an industrial suburb on the outskirts of Sao Paulo.

Acting on orders from imprisoned PCC leaders, they planned to shoot as many as 60 guards from four lockups over a 10-day period as they headed to work or finished their shifts, district police chief Marco Antonio de Paula Santos said.

"We believe they launched these attacks as a way to pressure authorities to relax the more severe prison conditions imposed on gang leaders after the May attacks," Santos said.

There were no signs that Monday's violence was spreading or having any impact on daily life in Sao Paulo, a metropolis of 18 million. Sao Paulo state Gov. Claudio Lembo said no new waves of widespread violence were expected.

But Santos said police were taking no chances and have been placed on maximum alert "to make sure they (the PCC) will fail if they decide to launch a new attack."

Officers learned about the plan to attack prison guards by listening in on gang leaders' phone calls and from informants, according to Santos and state police.

Before dawn Monday, undercover officers spotted the alleged PCC members getting ready to attack three guards who were leaving the Sao Bernardo prison and exchanged gunfire with the suspects.

Some suspects were killed at a gas station just 500 yards outside the prison, and the rest were shot dead in gunbattles on the streets as they tried to flee in cars, Santos said. One officer was grazed by a bullet, five suspects were taken into custody uninjured and at least four escaped. Authorities seized eight handguns and a rifle.

The violence came just six weeks after imprisoned gang leaders allegedly ordered attacks against police across the city and Sao Paulo state, touching off a weeklong wave of violence that killed nearly 200 police, prison guards, suspected criminals and jail inmates.

Last month's attacks terrified Sao Paulo, prompting businesses to shut down and parents to pull their children from schools after suspected gang members torched scores of buses and attacked other symbols of authority across the region.

Jailed members of the gang allegedly ordered the rampage using smuggled cellular phones because of outrage over a government plan to transfer gang leaders to more secure prisons. The May 12-19 spree left 41 officers and prison guards dead.

Police struck back, killing 123 people, many described as gang members, though human rights advocates said they suspected innocents were killed. Twenty-three inmates also died in prison rebellions.