By Brian Winter and Marcelo Teixeira

SAO PAULO (Reuters) - Brazilian police and protesters clashed on Thursday just hours before the opening game of the World Cup, injuring at least five people, although excitement began to build elsewhere as cheering, flag-waving fans converged on the stadium.

The tournament has been largely overshadowed so far 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.

Police fired tear gas, rubber bullets and noise bombs to disperse more than 100 demonstrators who gathered in eastern Sao Paulo, about six miles (10 km) away from the stadium where the opener between Brazil and Croatia is set to take place.

After protesters tried to cut off a main road to the stadium, at least five people were injured in clashes, a police spokesman said.

Photos circulated by a protest group showed a handful of protesters with injuries including bloody noses and what appeared to be wounds on their legs from rubber bullets.

About 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. There were smaller demonstrations in other host cities.

Elsewhere, though, including at the Corinthians arena where the opening game will start at 5 p.m. local time (2000 GMT), the somber mood that has permeated World Cup preparations in recent months showed signs of finally turning festive.

Fans at the stadium, most of them dressed in the canary yellow and green of Brazil's national team, chanted, sang and danced. In downtown Sao Paulo, crowds of Croatian fans were drinking beer by mid-morning.

Outside city hall, Tuany Ramos sang along with about 50 other fans setting off firecrackers and blowing air horns. "It finally arrived and we are very excited to cheer for Brazil," Ramos said.

The stakes are 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.

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Rousseff, who will attend the opening game, has dismissed complaints about overspending and delays in preparing stadiums and airports. Asked if the home team would win the opener, as it is expected to do, she smiled and flashed a thumbs-up.

As many as six police helicopters circled around the Sao Paulo stadium and hundreds of police were on hand to keep protesters away.

GRUMPY MOOD STARTS TO THAW

More than 1 million people joined in the protests last June, but most recent demonstrators have been much smaller, numbering just a few hundred people. Polls suggest that, despite continued misgivings about the World Cup's organization, many Brazilians will start to enjoy it once the goals start coming.

Brazil's team, led by exciting 22-year-old star striker, Neymar, is widely fancied to win a record sixth World Cup title.

In Salvador, another of the 12 cities that will host games, 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.

The Sao Paulo 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."

A rough tournament would likely cause Rousseff's popularity, already under pressure, to fall further. Polls show she now holds a lead of about 10 percentage points over her likely rival if the election 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 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 in Rio de Janeiro and Brad Haynes and Esteban Israel in Sao Paulo; Editing by Kieran Murray)