Touadera was first elected in 2016 and won reelection in 2020. He has been campaigning for the constitutional referendum which, if passed, would remove a two-term presidential limit and enable him run again.
Evariste Ngamana, head of the referendum campaign and vice president of the national assembly, said on Thursday that 80% of voters have been mobilised and called on the people to vote "yes".
However, the opposition coalition Republican Bloc for the Defense of the Constitution has asked Central Africans to boycott the vote and stay home.
"The Political Bureau of the URCS (Union for Central African Renewal) urges the entire population... to refrain from taking part in this gigantic swindle, not to go to the polling stations and to freely go about their Sunday activities," the opposition party said in a statement on Wednesday.
Opposition leader Joseph Bendounga also said he was campaigning for abstention in Sunday's referendum.
Touadera turned to Russia for help tackling rebel groups which occupy large swaths of territory. The country of around 5.5 million people which is rich in gold, diamonds, and timber has struggled to find stability since independence in 1960.
Russia first sent security contractors to CAR in 2018, and stepped up its support with more than 1,500 troops including instructors and soldiers fighting alongside the country's army. Insecurity has persisted.
Dozens of troops from Russia's Wagner private military company arrived in the country earlier this month to help secure the constitutional referendum, the presidency said.
(Reporting by Judicaël Yongo; Writing by Anait Miridzhanian; Editing by Nellie Peyton and Jonathan Oatis)
