Results: Brazil 1-0 Belgium, Australia 1-1 Serbia, Japan 0-1 USA, Honduras 0-3 Italy, Argentina 2-1 Ivory Coast, China 1-1 New Zealand, Holland 0-0 Nigeria, Korea 1-1 Cameroon
Cannot play media. Sorry, this media is not available in your territory.
Brazil struggled to open up the Belgium defence
Brazil midfielder Hernanes scored the winner as they beat nine-man Belgium in Group C on the opening day of the men's Olympic football tournament.
Belgium had Vincent Kompany sent off before Hernanes fired home from inside the box and Marouane Fellaini also saw red with two minutes left.
The United States top Group B after beating Japan and seeing Nigeria draw with the heavily fancied Netherlands.
Barcelona's Lionel Messi led 2004 winners Argentina to an opening win.
Messi was given a late reprieve after his Spanish club agreed not to recall him from the tournament, and he responded by scoring one and having in a hand in their other goal in a 2-1 victory against the Ivory Coast.
They lead Group A after Australia were held to a 1-1 draw by Serbia.
The ball bounces around all over the place and you need two or three touches to bring it under control
Australia had looked headed for an opening victory, with Ruben Zadkovich giving them a 1-0 lead in the 69th minute.
But Slobodan Rajkovic equalized with a header from a corner to earn a draw for Serbia.
Manchester United's Dong Fangzhou scored a last-gasp equaliser as China scraped a 1-1 draw against 10-man New Zealand.
The striker came off the bench to head in with two minutes left and send a packed crowd into a frenzy.
It was a crucial point with the hosts still to play Belgium and Brazil in the group.
Brazil had former world player of the year Ronaldinho in their starting line-up, but it was Sao Paulo's Hernanes who grabbed the glory when he twisted in the box and rifled a shot into the net in the 79th minute.
Ronaldinho, who signed for AC Milan during the summer after a season of problems with injury and his fitness, was kept in check by the Belgian defence and restricted to two free-kicks.
Ronaldinho admitted he was still short of his best form
"I haven't played an official football match for four months. I'm happy to return," said the Brazil captain, who has been included as an over-age player.
"It was a complicated game with the heat and everything else, but I'm happy with the win. We have absolute confidence that we are going to get better during the competition."
The Brazilians have won five World Cups but their men's team have never claimed gold at the Olympics.
Brazil coach Dunga, who has already complained about the food and training arrangements for his team, was unhappy about the pitch at the Shenyang Olympic stadium.
"If it had been better it would have been a much better match, regardless of the opposition," he said.
"The ball bounces around all over the place and you need two or three touches to bring it under control."
He also complained about the heat, adding: "It was like a sauna and that makes it more difficult."
Brazil were helped by Belgium's loss of Kompany and Marouane in the second half - both for second bookable offences.
"I think the referee has made the difference," said Belgium coach Jean Francois de Sart said afterwards.
Italy, who took bronze in 2004, went to the top of Group D after beating Honduras 3-0.
First-half goals from forwards Sebastian Giovinco and Giuseppe Rossi gave Italy the initiative and Robert Acquafresca added another after the break as they made an impressive start to their Olympic campaign.
Honduras should have pulled one back late in the game, but Carlos Pavon fired a penalty over the bar.
In the other Group D gam,e George Mandjeck scored late on to earn Cameroon a 1-1 draw with South Korea.
Scottish-born Stuart Holden, meanwhile, scored the winning goal for the US when he struck a loose ball from the edge of the area past Japanese goalkeeper Shusaku Nishikawa.
It was a crucial victory for the US, who saw their Group B rivals Nigeria and a powerful Netherlands side share a 0-0 draw.