(Adds color, Brazil victory in opening game, details)
By Brian Winter and Marcelo Teixeira
SAO PAULO, June 12 (Reuters) - Brazil exploded with street parties as its soccer team won the World Cup's opening game on Thursday but scattered violent protests were a reminder that many locals remain angry over the billions spent to host the tournament.
Millions of fans dressed in Brazil's canary yellow, green and blue home colors, cheered throughout Brazil's victory over Croatia in Sao Paulo and continued the revelry into the night.
The country briefly fell silent when Croatia took an early lead, but fireworks, horns and drum beats reached a crescendo as Brazil rallied for a 3-1 win.
Despite worries over traffic and the Sao Paulo stadium, which was completed six months late and wasn't fully tested before the game, there were no initial reports of major logistical snafus. About a quarter of the stadium's floodlights briefly went out during the game, but they soon turned back on.
"Despite all the controversy, this is the World Cup and we are Brazilians. We need to forget about all that now and cheer for Brazil," said Natia Souza, a fan in downtown Sao Paulo.
President Dilma Rousseff, who attended the game and has defended the Cup against criticism ahead of her bid for re-election in October, was repeatedly jeered by many in the stadium crowd and by fans at outdoor viewings across the country when she appeared on giant screens.
The tournament's run-up was largely overshadowed by construction delays and months of political unrest with many Brazilians furious over $11 billion being spent to host the Cup in a country where hospitals and schools are often poor.
Protests flared on Thursday in many of the 12 Brazilian cities that will host games, including Sao Paulo, Rio de Janeiro and Belo Horizonte. Some gathered more than 1,000 people, while others saw just a few dozen.
Late in the morning, police fired tear gas, rubber bullets and noise bombs to disperse about 600 demonstrators who gathered in eastern Sao Paulo, about six miles (10 km) away from the Corinthians arena where the game took place.
After protesters tried to cut off a main road to the stadium, six people were injured, including some journalists, a police spokesman said. Three protesters were arrested.
More than 10 were arrested in the southern host city of Porto Alegre, a police spokesman said. Demonstrators there overturned a police car and smashed bank windows.
Roughly 1,000 protesters in Rio de Janeiro marched peacefully, though some burned Brazilian flags and carried signs saying "FIFA go home," in a reference to the world soccer body. A Reuters photographer was hit in the head by a rock in Belo Horizonte. He was in stable condition.
Even some who weren't marching said they were still upset.
"Sadly these protests are too late. The money has already been robbed and there's nothing we can do now but cry," said Luiz Reis, 47, a lawyer on his way home to watch the game.
Elsewhere, though, the dour mood of recent months seemed to melt away.
Led by 22-year-old star striker Neymar, the team is widely fancied to win a record sixth World Cup - and its first on home soil. Neymar scored twice and was named man of the match in the win against Croatia.
In Salvador, another host city, locals were singing soccer songs and playing drums as others hung yellow and green streamers.
"You can feel the atmosphere building up with fans coming here in good spirits," said Ben, an English fan in the sweltering Amazon city of Manaus.
Yet the list of possible problems is long. In fact, hosting a successful tournament may ultimately prove harder for Brazil than winning it.
About a dozen disgruntled airport workers blocked a road outside Rio's international airport on Thursday morning, causing heavy traffic.
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.
A rough tournament would likely cause Rousseff's popularity, already under pressure, to fall further. 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 will also give clues as to how well it will do in two years, when it plays host to the Olympics. (Additional reporting by Paulo Prada and Rodrigo Viga Gaier in Rio de Janeiro; Fabiola Gomes, Brad Haynes, Alberto Alerigi, Caroline Stauffer and Esteban Israel in Sao Paulo, Neil Maidment in Salvador, David Ljunggren in Manaus and Andrew Cawthorne in Porto Alegre; Editing by Kieran Murray)