Police fired tear gas and swung batons to beat back the picketing strikers after commuters tried to enter a major metro station amid torrential rain that has added to the traffic misery.
The indefinite work stoppage, now in its second day, has raised fears of unrest when the country’s business hub hosts next Thursday’s game between Brazil and Croatia.
The clashes came as Brazil’s national team prepared to play its last friendly on Friday night against Serbia in Sao Paulo’s Morumbi stadium, not the new Corinthians Arena, which has yet to be finished six days from the inaugural match.
The Sao Paulo metro is the main link to the Corinthians Arena, and the strike could force organizers to come up with last-minute alternative transportation for tens of thousands of fans.
The traffic mayhem has stranded the 4.5 million passengers who use the subway system daily in the sprawling city, while bumper-to-bumper traffic stretched for up to 250 kilometers (155 miles).
“I’m going to have to return home. I can’t get to work like this. The metro is not going there and with this traffic, it’s impossible to go by bus,” said Pedro Henrique Rodrigues, who stood in a massive line of people waiting for buses.
Another strike by 75 percent of Sao Paulo’s traffic police exacerbated transport problems.
The chaos in Sao Paulo is the sort of incident that Brazilian and world football body FIFA want to avoid following the violent protests that marred last year’s Confederations Cup, a World Cup dress rehearsal.
It was in this city of 20 million people that mass protests erupted a year ago as citizens took to the streets to voice anger at rising public transport fares.
The unrest ballooned into nationwide demonstrations against the more than $11 billion being spent on the World Cup, with more than one million people taking to the streets to demand that money instead be spent on hospitals and schools.
Amnesty International said the police response to the protests had been characterized by violence and abuses, and warned Brazil against cracking down on demonstrations during the World Cup.
“Brazil’s deficient policing record, reliance on the military to police demonstrations, lack of training and an atmosphere of impunity creates a dangerous cocktail,” said Atila Roque, Amnesty’s country director for Brazil.
On Thursday, frustrated commuters broke entrance gates at the station that serves Corinthians Arena.
After some of them jumped onto the tracks, system operators CPTM decided to open the station in a bid to calm the situation.
Despite Sao Paulo’s latest troubles. FIFA president Sepp Blatter predicted on Thursday that tensions would subside once the football began.
“We at FIFA, we are confident, it will be a celebration,” Blatter said. “I’m an optimist. After the tournament kicks off I think there will be a better mood.”
FIFA secretary general Jerome Valcke insisted preparations were on track but acknowledged the opening weeks of the tournament would be the “most challenging.”
“We are in control, we have nothing to fear in the coming days,” Valcke said.
The protest movement has lost momentum since last June, but the transportation chaos risks rekindling anger in the countdown to the opening ceremony.
Workers went on strike early Thursday after negotiations on a salary increase fell through. They rejected an offer of 8.7 percent, insisting on at least 10 percent.
Talks between union leaders and subway management ended on Thursday with no deal.
President Dilma Rousseff has defended her government’s preparations, insisting the money spent will leave a legacy of airports and transport infrastructure that will benefit Brazil for years to come.
But the government has also faced criticism for chronic delays and disorganization.
Workers are still scrambling to finish several of the 12 host stadiums, while the Corinthians Arena has not received safety clearance from firefighters to operate at full capacity.
Eight workers died in construction accidents at the stadiums, including three in Sao Paulo.
Much of the other promised infrastructure has been shelved, from roadworks and a high-speed train to subway and monorail lines.
Meanwhile, teams continue trickling into Brazil. Chile’s arrival on Thursday followed that of Australia, Croatia and Iran.