Over 2,000 candidates are running for 151 assembly seats in the vote that began on July 10, according to the country's directorate general of electoral affairs.
The councilors chosen by voters on Sunday will in turn appoint the 72 senators in the first chamber of parliament.
The Congolese Labour Party (PCT) of President Denis Sassou Nguesso is currently the largest in the 151-seat assembly, with 101 deputies.
Nguesso, a 78-year-old former paratrooper, has been president of the Republic of Congo for a total of 37 years.
The country's leading opposition party, the Pan-African Union for Social Democracy (UPADS), is fielding 45 candidates.
UDH-Yuki, another opposition party, is putting forward 32 candidates.
But two small opposition parties have said they boycotted the election over fears that it has be rigged.