Mr Sassou-Nguesso is widely expected to win another seven-year term
People in the Republic of Congo have voted in an election which opposition leaders boycotted over allegations it would be neither free nor fair.
President Denis Sassou-Nguesso, who has been in power for most of the past 30 years, was widely expected to win another seven-year term.
Turnout was fairly low, but voting was peaceful, election observers said.
Opposition leaders had urged voters to stay away, saying the government had inflated the electoral roll figures.
The BBC's Thomas Fessy in the capital Brazzaville says many people have left the city fearing unrest.
Voting was slow, with no queues reported at polling stations in several parts of the country, he says.
Before the polls closed, the head of the African Union observers, Dieudonne Kumbo Yaya, told the BBC his team was not aware of any fraud having taken place.
But our correspondent witnessed money being handed out at a polling station in the south of the capital, to people who later said they had been asked to vote for Mr Sassou-Nguesso.
"They are giving money and they tell us for whom to vote for," one voter told the BBC.
"As abstention is really high, they ask for your voting card and then give you money to go vote, this is what they are doing at the moment."
Another man said he had not voted because he had not been given a polling card.
Opponents said the electoral roll had been rigged
Earlier in the day, Mr Sassou-Nguesso told a rally of his supporters in the capital: "Fear not and go and vote. There will not be any more war in Congo."
But his main rival, Mathias Dzon, and four other candidates had urged voters to boycott the polls.
"No-one should go and vote on Sunday. Stay at home - we don't want an electoral hold-up or a parody of an election," said Clement Mierrassa, head of the opposition Congolese Social Democratic Party.
Government officials say more than two million people have been registered to vote but Roger Bouka Owoko, head of the Congolese Observatory for Human Rights (OCDH), said that figure was "grotesque".
"Congo cannot have so many electors," he said. "This monstrous electoral register is the drawback of the electoral process."
One election observer, speaking on condition of anonymity, told Agence France Presse: "There are more observers than voters".
The head of the European Commission delegation in Congo, Miguel Amado, said he also had concerns about the electoral roll being used and said many people had not received voting cards.