Police had earlier fired tear gas at hundreds of opposition activists who converged near the Constitutional Court in the capital, Libreville, as army helicopters flew overhead.

Looting and clashes also followed Bongo's win in 2009, when he came to power after the death of his father, longtime ruler Omar Bongo.

Bongo won this election with 49.8 percent of the vote, while Ping had 48.23 percent. The constitutional court must finalise the electoral commission's provisional results.

The results came a day later than expected, prompting fears of a tainted process.

European Union observers criticised a "lack of transparency," and the EU called for the electoral commission to publish the results from all polling stations.

France and the United States also called for individual polling station results to be published. The US Embassy said 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."

UN Secretary-General Ban Ki-moon urged political leaders and their supporters "to refrain from further acts that could undermine the peace and stability of the country" and urged security forces to "exercise maximum restraint in response to protests," his spokesman said.

Ban also called on political leaders "to address any disputes they may have through existing constitutional and legal channels," spokesman Stephane Dujarric said.

The secretary-general is sending his special representative for central Africa, Abdoulaye Bathily, to Gabon to help in efforts to "calm the situation and to peacefully resolve the contentious issues emanating from the electoral process," Dujarric said.

The US State Department also urged calm and encouraged security forces to "act with both restraint and respect for the human rights of all Gabonese citizens."

Bongo, in a statement after his victory, congratulated everyone for voting in peace and transparency. "We are living in a moment where we must be more solid and more strong together," he said.