Gabon's most high-profile opposition candidate has warned of possible tampering with the outcome of Saturday's presidential election, potentially increasing tensions as this central African nation awaits an official tally.

Jean Ping, a former chair of the African Union Commission, is trying to unseat President Ali Bongo Ondimba and topple a family dynasty that dates back to the 1960s. Bongo came to power in 2009 after the death of his father, longtime ruler Omar Bongo.

"We are well aware of all the clear manipulations and other modifications ... underway at the Autonomous and Permanent National Electoral Commission and the interior ministry," Ping said at a press conference Monday. He repeated the claims he made over the weekend that he was the winner, calling on Bongo to step down to allow for "the transfer of power."

Bongo's campaign, meanwhile, has predicted the president will win a second seven-year term.

Official results are expected Tuesday, and Gabon's interior ministry has said it is illegal to pre-empt them. On Monday, U.N. Secretary-General Ban Ki-moon's spokesman issued a statement urging political stakeholders in Gabon to "refrain from making remarks on the outcome of the election before it is known."

Ten candidates competed to lead the oil-producing nation of about 1.5 million people. Gabon does not have a runoff system, meaning whoever gets the largest share of Saturday's vote will be the winner.

Bongo's victory in the 2009 vote sparked looting and clashes between protesters and security forces.