Hundreds of rebels have entered the Central African Republic (CAR) capital Bangui, according to witnesses.
Seleka rebels were said to be fighting running battles with government troops.
The fighters, who have been involved in an on-off rebellion since December, accuse President Francois Bozize of failing to honour a peace deal.
Former colonial power France has called for an emergency meeting of the UN Security Council, and reportedly sent troops to secure the airport.
French officials warned French nationals in the country should stay at home.
Rebel spokesman Nelson Ndjadder said they had shot down a government military helicopter and were now heading for the presidential palace.
However, there were no further reports of fighting as darkness fell.
The rebels are also said to have cut off electrical power to parts of the city, having taken control of three power plants in the neighbouring town of Boali.
A local UN official in southern Bangui said people were in a state of panic but could not confirm the rebels had entered the city.
The rebels joined a power-sharing government in January after talks brokered by regional leaders to end a rebellion they launched last year.
But the deal quickly collapsed, with the rebels saying their demands, including the release of political prisoners, had not been met.
On Friday it was reported that they had taken a checkpoint in the town of Damara, about an hour's drive from Bangui, where regional Fomac peacekeepers are based.
BBC Africa editor Richard Hamilton says government soldiers have been unable to fend off the rebels because Mr Bozize fears being overthrown in a coup and is therefore wary of having a strong army.
He came to power himself in a military coup in 2003.
CAR has been hit by a series of rebellions since independence from France in 1960.
It is one of the poorest countries in Africa, despite its considerable mineral resources.