Beijing - Sergio Aguero scored twice and Juan Riquelme converted from the penalty spot as defending champions Argentina beat arch-rivals Brazil 3-0 on Tuesday to book their place the final of the Olympic football tournament.

Brazil finished with nine men after first Lucas and then substitute Thiago Neves were sent off for fouls on Javier Mascherano as the five-time world champions continue their wait for a first ever Olympic gold in men's football.

Sergio Batista's Argentina side will now meet Nigeria in Saturday's final after the 1996 Olympic champions defeated Belgium 4- 1 in the day's other semi-final in Shanghai thanks to a brace from Chinedu Obasi Ogbuke sandwiched in between goals from Olubayo Adefemi and Chibuzor Okonkwo.

"Brazil are strong opponents and the reason why we won today is because we played to our own style and to our strengths," said the Argentina coach.

"We knew Brazil would be strong and that is why we played a patient game."

Patience was certainly the order of the day in the first 45 minutes at the Workers Stadium in Beijing as both sides went in at the break scoreless with chances having been at a minimum.

However, the warning signs were there as Lionel Messi's tireless running caused panic in the Brazil defence on several occasions during the first period, not least five minutes before the break when he dribbled to the byline but his low cross failed to find a teammate.

The closest Brazil came to scoring was through a long-range shot from Hernanes, which fizzed over.

The second half was a completely different affair and within seven minutes of the restart Argentina were in front through Aguero, albeit controversially when the Atletico Madrid striker appeared to steer Angel Di Maria's cross in with his upper arm.

Brazilian protests came to nothing but within two minutes they could have been level. A cross by Rafinha nearly found Ronaldinho but the clearance only went as far as Rafael Sobis whose shot came off the foot of the post.

Four minutes later the Selecao found themselves two down as Aguero tapped home a Ezequiel Garay cross after some superb build-up work from Messi.

Ronaldinho hit a post for Brazil on 63 minutes before Riquelme sank their rivals with a third from the penalty spot after Breno fouled Aguero in the area.

Brazil finished the match in total disarray as first Lucas on 81 minutes and then Neves three minutes later saw red cards for fouls on Mascherano.

"In a game between Brazil and Argentina the side that leads will have a huge advantage," said a disappointed Brazil coach Dunga.

"I think Argentina played very well in this match and were very tough. This is what they are good at."

Dunga refused to be drawn on his side's abysmal performance or on Ronaldinho's lacklustre showing, admitting only that his team had not managed to contain Messi.

Batista, on the other hand, was more than happy to point out that his side's opponents had failed to trouble Argentina.

"Brazil lacked a cutting edge in attack in the second half, that's why things got a bit rough at times but I wasn't worried as that's football," said Batista.

Earlier, Nigeria clinched their place in the final with a convincing 4-1 victory against Belgium in the Shanghai stadium.

The 1996 champions, who beat Argentina in the final in Atlanta to become the first African country to win a gold medal in Olympic football, were in total control after taking the lead through Adefemi in the 18th minute.

Ogbuke, whose Bundesliga club Hoffenheim had called on him to return to Germany after the Court of Arbitration for Sport ruled that clubs did not have to release their players for the tournament, added a second on the hour.

The striker scored his brace with a long range effort from 30 metres that gave goalkeeper Logan Bailly no chance in the 72nd minute.

Okonkwo made it four for his side just six minutes later before Ciman Laurent pulled one back for the Belgians two minutes from the end.

Adefemi said that his side had been under pressure. "Seeing what we achieved in Atlanta in 1996, everyone was waiting for us to do just the same. But we held on really well and won."

Nigerian coach Samson Siasia is now optimistic that the side can go on to win the gold. "We are going for gold. It was unbelievable."

Meanwhile, Batista said his side would relax first before looking towards the Nigeria clash.

"There is no doubt that Nigeria is a tough team but we must just enjoy today and now rest," he said.

"I think this Argentina team tries to go for victory in every game and wants the gold medal so that is what they will try and do against Nigeria."