The decision, which the source said was taken for reasons of security, will cause inconvenience and possible alarm in a country where there are few landlines and many rely on cell phone communication. It is also highly unusual in the context of an election in Africa.

"The Minister of the Interior indeed signed and sent a letter to ... MTN Congo and Airtel Congo to tell them that the state wants them to cut off communication on March 20 and 21 for reasons of security and public tranquillity," the source told Reuters.

President Denis Sassou Nguesso is expected to win the election and extend his long rule and on Friday he urged voters to give him a first round victory.

Sassou Nguesso, who has led Congo for 32 of the last 37 years, pushed through constitutional changes last October to remove term and age limits that would have prevented him from seeking re-election.

Eight opponents are running against Sassou Nguesso, including retired General Jean-Marie Mokoko, who once served as the president's security adviser.