Violence has broken out in the Gabonese capital Libreville after President Ali Bongo was declared winner of Saturday's presidential election.
Opposition candidate Jean Ping said two people had been killed and many injured after security forces launched an attack on his party's headquarters.
The government said it was targeting "armed criminals" who had earlier set the parliament on fire.
Mr Ping disputes the result, which gave President Bongo a narrow victory.
"They attacked around 01:00 (00:00 GMT). It is the republican guard. They were bombarding with helicopters and then they attacked on the ground," Mr Ping - who was not at the party headquarters himself - said earlier.
He called for international assistance to protect the population.
The election result was announced on Wednesday afternoon and gave Mr Bongo a second seven-year term with 49.8% of the vote to Mr Ping's 48.2 %, a margin of 5,594 votes.
Mr Ping said the election was fraudulent and "everybody knows" he won.
Protesters took to the streets shortly after the announcement. They set fire to the parliament building and clashed with riot police.
Mr Ping's camp has said figures from the president's stronghold showed a 99% turnout. He has called for voting figures from each polling station to made public.
The US and EU have also called for the results to be made public while UN Secretary-General Ban Ki-moon has urged calm.
In 2009, Mr Bongo took over from his father, who came to power in 1967.
Before entering politics Mr Ping was a career diplomat who served as the chairman of the African Union Commission.