Congo is Africa's fifth-biggest oil producer and investors are concerned that the vote could trigger a repeat of the conflict and rows that have marred previous elections.
But 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 is one of three African presidents whose wealth is being investigated by a French judge.
He 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.
"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, the head of the Congolese Social Democratic Party, which is part of the coalition calling for a boycott.
More than 2.2 million people will be eligible to vote for the 13 candidates, the election commission has 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.