Despite protests in which at least 18 demonstrators died, Sassou Nguesso pushed through constitutional changes in October to remove term and age limits that would have prevented him from standing again.
Police fired tear gas at opposition supporters in Congo Republic on Sunday, witnesses said, after voting ended in a poll expected to see long-time leader Denis Sassou Nguesso extend his three-decade rule.
The government ordered mobile phone and internet services cut for the day across the oil-producing Central African country "for reasons of security and public tranquillity", a government official said. It also banned motor vehicle use nationwide.
Despite protests in which at least 18 demonstrators died, Sassou Nguesso pushed through constitutional changes in October to remove term and age limits that would have prevented him from standing again. He is now heavily favoured to win the polls.
He faces eight opponents, including retired General Jean-Marie Mokoko, seen as the strongest challenger.
"I want this to go well. I don't want war, which is often what happens after these elections," said Damien Kiongazi, who returned home to the capital Brazzaville from Paris to vote.
However, soon after polling stations closed, security forces moved in on crowds that had gathered in the capital's Bacongo neighbourhood, an opposition stronghold.
Witnesses who said they had been following the vote counting were then teargassed by riot police. A heavy odour of the gas still hung in the air when a Reuters reporter arrived in the neighbourhood.
Sassou Nguesso, who ruled from 1979 until he lost an election in 1992, regained power in 1997 after a brief civil war and then went on to win disputed polls in 2002 and 2009.
His supporters credit him with restoring stability and developing the country's infrastructure.
"I think the vote marks progress for our democracy. And I can say that the new republic is setting out under a good omen," Sassou Nguesso said after voting in Brazzaville.
The president's critics claim Congo's oil wealth has enriched and entrenched a small elite, while around half of the population of 4.5 million lives in poverty.
The polls will be watched closely by other leaders in Africa - notably in neighbouring Democratic Republic of Congo - because several long-serving presidents are seeking to stay on beyond constitutionally mandated term limits.
The opposition said on Friday it had documented preparations for widespread vote rigging, including voters registered at multiple polling sites, individuals with multiple voter cards and the distribution of voter cards to non-citizens.
The government rejected accusations it was preparing to cheat, claiming its opponents were laying the groundwork for post-election chaos.
Dileita Mohamed Dileita, a former Djibouti prime minister heading the African Union's elections observer mission, said around noon that he believed the polls were "going very well".
However, at a polling station in Mafouta in southern Brazzaville, some of those waiting to cast their ballots on Sunday morning complained that the posted voter list contained the names of a number of people who had died years before.
At the same location, a Reuters reporter saw three names repeated twice, listing the same birthdays and parents.
"I'm not confident. I see already that our voices are being stolen. The real results will not be given," said Boclelon Ganga, 28, as he smoked a cigarette while waiting to vote.
Candidate Brice Parfait Kolelas said the opposition would use satellite phones to circumvent the communications blackout in order to monitor the results.
"We are making a compilation," he said. "I don't encourage taking to the streets, but people need to protect their vote."
Former colonial power France criticised the conditions of October's constitutional referendum, saying they did not allow an accurate assessment of the result. The European Union has decided not to send a mission to observe Sunday's vote.