The oil-rich central African country has been open to multiparty politics since 1991 but wracked by two civil wars in which Sassou Nguesso, an army colonel who first came to power in 1979, played a prominent role.
Voting got off to a late start in some parts of Brazzaville, but Sassou Nguesso, who cast his vote at midday near the presidential palace, sought to reassure the nation that everything was proceeding smoothly.
"The instructions I had given for the elections to take place in peace, transparency, for them to be free, fair and credible, have for the most part been followed," he said.
More than two million people are eligible to vote in the first round of the poll, but the dominance of Sassou Nguesso's Congolese Labour Party (PCT) and its allies means there is not much election-day suspense in a country where half of the four million people live below the poverty line.
Turnout was low at midday but an international election observer said she believed the pace would pick up later in the day.
"We suppose people haven't yet gone out," said Martine Logbo of GIDSE, an Côte d'Ivoire-based election monitoring group.
Election commission chief Henri Bouka said "everything is going normally", though earlier in the day he had said some polling stations experienced equipment problems.
Vying for seats
Simone Moukouli, a 51-year-old voter, expressed frustration that "things got started too late".
"It's our duty to put in place the institutions of the republic, without which the country can't function," Moukouli said.
In all, more than 1 200 candidates including 124 women are vying for 135 seats.
The PCT has established a dominant role in the Congo through alliances formed with smaller parties and "independent" lawmakers who are close to it.
The opposition held just 12 seats in the outgoing legislature.
To confront the PCT, the main opposition Panafrican Union for Social Democracy (UPADS) – which held 11 seats in the outgoing Parliament – has put up 70 candidates.
UPADS was formerly led by ex-president Pascal Lissouba, ousted in 1977 by Sassou Nguesso, who later proclaimed himself president.
The other opposition party with a seat in the outgoing parliament, the Union for the Republic and Democracy, hopes to do better than in 2007, when it won only one seat.
Abusing state resources
An opposition coalition that boycotted the 2007 poll, the Alliance for the Republic and Democracy, is represented by about 70 candidates.
Finally, a former rebel movement, the National Council of Republicans (CNR), headed by Frederic Bintsamou, alias Pastor Ntumi, is fielding 20 candidates.
On Sassou Nguesso's side, the president has a son and daughter both running on the PCT ticket: Denis Christel Sassou Nguesso, a director at state-owned oil company SNPC, and Claudia Lembouma Sassou Nguesso, who is communications advisor to her father.
During the campaign, opposition parties and civil society groups accused the ruling coalition of abusing state resources, particularly the public media.
The last parliamentary elections in 2007 were marred by irregularities and fraud denounced by the African Union, which has sent about 30 observers to monitor the poll, along with the Economic Community of Central African States.
Polling is not taking place in three districts of Brazzaville hit by a munitions dump explosion in March that killed nearly 300 people.
The official results due to be announced by Tuesday. A second round of voting is due on August 5. – Sapa-AFP