A total of 92.96 per cent of voters approved the constitutional change, which has now been adopted, Interior Minister Raymond Mboulou said.
Official results showed turnout was high at 72.44 per cent, though yesterday opposition leader Pascal Tsaty Mabiala had said only 10 per cent of Congolese voted.
According to reports by several AFP journalists in the capital Brazzaville, second city Pointe-Noire and several other areas of the central African country, people largely avoided voting offices.
The referendum proposed two changes to the constitution, which currently disqualifies Sassou Nguesso from running for re-election in 2016 because it stipulates a maximum age of 70 for presidential candidates and limits the number of mandates to two.
Sassou Nguesso is over the age limit and has already served two consecutive seven-year terms.