Polling station were reportedly busy in Sassou-Nguesso's strongholds, such as the northern parts of the capital city and the town of Makoua, about 600km away.
But in opposition areas people appeared to have heeded the call to stay away.
At one location in southern Brazzaville as few as 52 of 924 registered voters reportedly cast their ballots.
Congo is Africa's fifth-biggest oil producer and investors were concerned that the vote could trigger a repeat of the conflict and rows that have marred previous elections.
But Roger Bouaka, executive director of Congolese Human Rights Observatory which was monitoring the polls, said that there appeared to have been no irregularities or violence.
"The vote is taking place normally, without any incidents in any of the areas that we have sent observers."
Critics say that only a small elite of Congo's four million people has benefited from oil revenue, despite large amounts of oil being produced.
|Sassou-Nguesso has been in and out of power since a coup in 1979 [AFP]
"I have just voted. I hope that the president will win. But if he is re-elected, he must tackle the problem of unemployment, increase teachers' salaries," Georges Itoua, 30, said.
Sassou-Nguesso has been in and out of power since a 1979 coup, losing multiparty elections in 1992 before coming back to power in a war that destroyed much of the capital in 1997.
The president won the last election in 2002, when his main rivals were banned or withdrew, citing irregularities.
Opposition parties had called for Sunday's vote to be postponed to allow for the creation of a new election commission and the clean-up of voter lists, which were a source of complaints during 2002 polls.
More than 2.2 million people were eligible to vote, the election commission said.
But many say they have not been issued with their cards while opposition parties say an extra 500,000 that were printed were fraudulent.