Libreville (AFP) - Angry protesters set fire to Gabon's national assembly on Wednesday as thousands of people took the streets after an announcement that President Ali Bongo had been re-elected, witnesses said.

"The whole building is catching fire," said a man at the scene called Yannick.

AFP journalists further away saw a plume of fire and smoke rising into the air above the building.

"They got in and burned it," Yannick told AFP, saying the security forces had pulled back from the area. They had been deployed there since Tuesday afternoon, when the results were initially due to be published.

The national assembly lies on the same road as several important institutions, among them the senate, the oil ministry, several embassies and the headquarters of state television.

The clashes erupted as soon as Bongo was declared the winner of Saturday's presidential poll, with opposition supporters chanting "Ali must go!"

As chaos erupted on the streets outside, Bongo hailed the outcome of the election, which he declared had been "peaceful and transparent" despite the opposition crying foul.

"I want to reiterate our primary victory: this election was exemplary," he said in his first remarks since the result was announced.