While no fresh violence is expected, opposition parties have cried foul, as over two million voters are expected to cast their ballots in the first round of polling to elect National Assembly members as well as local councils.
Sassou Nguesso returned to office in March last year after a constitutional referendum ended a two-term presidential term limit, amid deadly violence notably in the Pool region neighbouring the capital Brazzaville.
On Sunday, the 73-year-old president's ruling Congolese Labour Party (PCT) is running 128 candidates for the 151 seats available, while several independent candidates have close ties to the party.
The main parliamentary opposition group, the Pan-African Union for Social Democracy (UPADS), has put forward 43 candidates, compared with 31 run by the UDH-Yuki group of Guy-Brice Parfait Kolelas, who came second in the presidential elections last year.
But an opposition coalition comprising the IDC, the FROCAD and the CJ3M has said it will only take part under certain circumstances, including "the end of the crisis in Pool and the release of all political prisoners".
Two leaders of the coalition, Jean-Marie Michel Mokoko and Andre Okombi Salissa, were arrested and jailed in June 2016 and in January this year "for threatening the internal security of the state".
However, the head of the Independent National Election Commission (CNEI), Henri Bouka, insisted voting will be held nationwide, including in Pool.
"No [constituency] will be left out," said Bouka.
UPADS spokesman Daniel Tsoumou Ngouaka said the ruling party had an unfair advantage.
"It's a one-way campaign because the PCT has controlled all the state media, leaving no space for others," said Tsoumou.
"The PCT drew on public funds to the campaign of its candidates and its allies. It's unacceptable," added Christophe Moukoueke, spokesman for a collective of opposition parties which has called for a boycott of Sunday's polls.
Sassou Nguesso, a former paratrooper, served as president from 1979 to 1992, returning to power in 1997 following a civil war. He won two successive terms in elections in 2002 and 2009, both of which were disputed by opposition parties.
Congo has substantial deposits of oil, timber and diamonds, but oil production remains its most important economic sector, and Congo is the fourth-biggest producer in sub-Saharan Africa.
Dwindling reserves and civil wars that have ravaged the economy has slowed growth. Almost half the population of 4.5 million people still live in poverty, and according to World Bank data.
The second round of legislative polls will be held on July 30.
Source: News agencies