Violence has broken out the Gabonese capital Libreville after President Ali Bongo was declared winner of Saturday's presidential election.
Opposition candidate Jean Ping said security forces had bombarded his headquarters using helicopters before attacking on the ground, leaving 19 people injured.
Earlier his supporters set the national assembly on fire and fought police.
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. There are 19 people injured, some of them very seriously," said Mr Ping, who was not at the party headquarters himself.
"For nearly an hour the building has been surrounded. They want to enter the building. It is extremely violent," he added, AFP news agency reported.
Image captionSupporters of Mr Ping clashed with police
Image captionSoldiers were called in to tackle protesters
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 has said the election was fraudulent.
His camp has said figures from the president's stronghold showed a 99% turnout. Mr Ping 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.
Image captionBefore entering politics Mr Ping was a career diplomat
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