LIBREVILLE, Gabon — Security forces fanned out across Gabon’s capital and residents stockpiled food on Tuesday as the central African country awaited results of a hotly contested presidential election.

The most prominent opposition candidate, Jean Ping, was looking to defeat incumbent President Ali Bongo Ondimba and topple a family dynasty that stretches back to the 1960s. Bongo, 57, came to power after the death of his father, Omar Bongo, who ruled Gabon for more than 40 years.

Ping, 73, has spent the three days since Saturday’s vote predicting victory and calling on Bongo to step down, while Bongo’s camp has said the president is sure to win.

Each side has accused the other of fraud. As officials delayed the announcement of results Tuesday night, voters said they were worried the process had been compromised.

“This long wait is suspicious. We have the impression that the government is trying to manipulate the results,” said Emile Ngou, a Ping supporter.

The U.S. Embassy in Libreville said in a statement on its website Tuesday that Gabon’s voters were not “well served by the many systemic flaws and irregularities that we witnessed,” including the late opening of polling stations and “last minute changes to voting procedures.” The embassy said the government should publish results by polling station.

European Union observers have criticized a “lack of transparency” on the part of institutions organizing the vote.

The office of U.N. Secretary-General Ban Ki-moon expressed concern on Tuesday about “premature declarations” and said any disputes arising from the announcement of results should be addressed through “existing legal means.”

Gabon’s interior ministry has repeatedly accused Ping of trying to destabilize the country by pre-empting official results. On Tuesday night, a government spokesman said there was evidence Ping had collaborated with an adviser to Ivory Coast President Alassane Ouattara in a plot to convince members of the electoral commission to resign. The accused adviser, Mamadi Diane, denied the allegations to the magazine Jeune Afrique, but Ouattara’s office announced late Tuesday that Diane had been removed from his post.

Bongo’s election win in 2009 sparked looting and clashes between protesters and security forces.