A yes vote will allow 72-year-old President Denis Sassou Nguesso of the Republic of the Congo to run for a third term, although he's already been in power for more than 30 years.
Tension remains high after four people were killed in protests against the vote and opposition activists fear it will be rigged in his favour.
Police placed a main opposition leader under house arrest on Friday. Earlier in the week, internet services were switched off before a main opposition march.
Leaders from West Africa met in May to reject a proposal allowing African leaders to continue to rule for decades on end.
What's driving many African heads of state to avoid retirement? And what are the implications for their people?
Presenter: Hazem Sika
Thierry Moungalla - Congolese minister of information
Matlotleng Matlou - Executive director of Excelsior Afrika Consulting
Lucas Olo - Regional director for Central Africa at Transparency International
Source: Al Jazeera