Police fired noise bombs to disperse a crowd of about 200 demonstrators angry about government overspending on the event. The protesters were trying to cut off a key avenue leading to the Corinthians Arena where the soccer match will be played on the eastern edge of Sao Paulo, a Reuters witness said.
At least one protester was arrested, local media reported. A producer for CNN was injured during the confrontation, witnesses said.
The protests were expected to grow in size before Brazil's team plays Croatia at 5 p.m. (2000 GMT).
Much of the rest of Brazil's biggest city and business capital resembled a ghost town during the usual morning rush hour after officials declared a partial holiday to ensure traffic to the stadium would be light. About 20 million people live in the metropolitan area.
Stakes will be high not just on the soccer field. Whether the tournament goes smoothly may also have an effect on President Dilma Rousseff's chances for re-election in October, as well as Brazil's flagging reputation among investors.
Many Brazilians are angry over the $11.3 billion spent on hosting the World Cup when basic social services are poorly financed. Their pessimism has so far overshadowed a brighter mood among the some 800,000 foreign tourists expected to come to Brazil for the event.
Melisa da Silva, who was wearing Brazil's green and yellow colors as she headed to work on the subway on Thursday, said the country might finally cheer up once play gets under way.
"Well, it's here, and I think now it's time to cheer the team," she said. "I don't see why people should still be sad."
Rousseff has dismissed complaints about the heavy spending and delays in preparing stadiums and airports, and is betting Brazil will put on a show on and off the field.
"What I'm seeing more and more is the welcome given to the teams and the happiness of the Brazilian people with our team," she said in a speech on Wednesday.
Brazil is widely considered the spiritual home of global soccer, and in recent days more of the flags and street parties that usually characterize World Cups here have begun to show up.
Yet the list of possible problems is long. In fact, hosting a successful tournament may ultimately prove harder for Brazil than winning it.
STADIUM A SOURCE OF ANXIETY
The main risk, for both fans and the government, appears to be violent street demonstrations.
Protests and labor strikes are planned in the 12 host cities, including a 24-hour slowdown by some airport workers in Rio de Janeiro, although the threat of a long subway strike in Sao Paulo has eased.
About a dozen disgruntled airport workers blocked a road outside Rio's international airport on Thursday morning, causing heavy traffic, local media reported.
Some businesses in Rio, the venue for seven Cup games, including the final, had boarded up windows and doors by late on Wednesday in case protests erupted.
The stadium itself has been a source of anxiety.
Not only was it delivered six months late at a cost of $525 million, about $150 million over budget, but because of the delays Thursday's game will be the facility's first at full capacity. That's a big no-no in the field of logistics and a violation of FIFA's normal protocol for World Cup games.
"I'm praying that nothing goes wrong," said Lizbeth Silva, a clerical worker at a Sao Paulo school. "You hear about all these problems, but you still want to root for Brazil."
Rousseff is running for re-election in October, and a rough tournament would likely cause her popularity, already under pressure, to fall further. Polls show she currently holds a lead of about 10 percentage points over her likely rival if the vote goes to a second round, as most expect.
Any major logistical problems and unrest could also further dent Brazil's reputation among investors, which has suffered since a decade-long economic boom fizzled under Rousseff.
Brazil's performance in hosting the World Cup may also give clues as to how well it will do in two years, when it plays host to the Summer Olympics.
At least one element is expected to cooperate on Thursday: the weather. Forecasters expect clear skies and a high of 75 degrees (24 C) - warm for the Southern Hemisphere winter.
Brazil's team, led by its exciting 22-year-old star striker, Neymar, is heavily favored to beat Croatia.
© Thomson Reuters 2014