Opposition candidate Jean Ping said the President Ali Bongo rigged the presidential elections and called for the international community "to help the people of Gabon."
Gabon's re-elected president, Ali Bongo, came under international scrutiny on Tuesday as a European Union mission questioned the validity of his narrow win, France recommended a recount and the African Union said it would send mediators.
Opposition leader Jean Ping, who has said the election was stolen, called on "the international community to help the people of Gabon," telling Reuters on Tuesday: "Everybody knows the result and everybody knows that Bongo is doing everything not to accept it."
Ping has said the number of votes cast in southeastern Haut-Ogooue province were inflated to give victory to Bongo, whose family has ruled the central African oil-producing country for almost half a century.
At least six people died in riots in the capital, Libreville, and other cities in the days after the announcement of results from the Aug. 27 election, which gave Bongo the victory over Ping by about 5,000 votes. Calm has since returned to the streets.
Ping said on Tuesday that between 50 to 100 people were killed since last week in Libreville. There was no independent confirmation of the figure.
Election monitors have focused on Haut-Ogooue, a Bongo stronghold, where official figures showed he won 95.46 percent of the vote on a 99.9 percent turnout.
The EU observer mission said the number of non-voters and blank or invalid ballots were at variance with the reported participation rate, adding turnout in other regions was around 48 percent.
"The integrity of the provisional results for this province is consequently put into question," said Mariya Gabriel, the EU's chief observer of the polls.
A government spokesman told Reuters he would not comment on the EU statement until Wednesday.
Opposition parties in Africa frequently say votes are rigged, but the results are rarely overturned and it is unusual for a president once declared winner, as in this case, to face significant international pressure over the election.
The African Union said it would send a delegation to Gabon likely to be led by Chad's Idriss Deby, one of Africa's longest-ruling presidents and the current chair of the pan-African body.
Ping, a former diplomat and African Union Commission chairman, said he had been told the delegation would arrive on Thursday.
He said he had no faith in the constitutional court because it was tied to the Bongo family and he wanted a recount done under international supervision before any appeal to that court.
Manuel Valls, prime minister of former colonial power France, suggested a recount would be wise and urged authorities to help locate about 15 of its nationals - out of a local French community of around 14,000 - it says are missing.
The government has dismissed all calls to publish more detailed results, prompting the justice minister to resign.