(Adds quotes, post-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, with a heavy presence of police and troops to maintain order.
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 reports of major logistical problems before or after the game.
Brazil's coach, Luiz Felipe Scolari, after the game praised the stadium as "incredible" and "fantastic."
The smooth first game, and especially the victory, raised the spirits of many who feared the worries of the past year could spoil the party.
"Despite all the controversy, this is the World Cup and we are Brazilians. We need to forget about all that now and cheer," 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 jeered by many in the stadium crowd and by fans at big-screen viewings across the country.
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.
Activists and human rights groups criticized some of their tactics. One protestor, already hit by rubber bullets, was photographed in a choke hold by one police officer while another blasted pepper-spray in his face.
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.
On the city's Copacabana beach, where thousands gathered for an outdoor broadcast, protesters hurled rocks at a studio set up by a British television crew.
In the southeastern city of Belo Horizonte, a Reuters photographer was hit in the head by a rock. He was in stable condition.
Elsewhere, the dour mood of recent months lifted.
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 sang soccer songs and beat 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, boarded up windows and doors in case vandals emerged with any protests.
A rough tournament would likely cause Rousseff's popularity, already under pressure, to fall further, even if the mood, for now, has improved.
Stressing his desire to have fun during the Cup, Leandro Aguiar, a 25-year-old fan in Sao Paulo, said: "It's too late to complain. In October we'll decide what's good and what's bad."
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 as host will also give clues as to how it might do in two years, when Rio hosts the Olympics. (Additional reporting by Paulo Prada in Rio de Janeiro; Brad Haynes and Caroline Stauffer in Sao Paulo, Neil Maidment in Salvador, David Ljunggren in Manaus and Andrew Cawthorne in Porto Alegre; Editing by Kieran Murray)