Hundreds of rebels have entered the Central African Republic capital, Bangui, and diplomatic sources say France has sent troops to its former colony to secure the airport.
Seleka rebels are said to be fighting running battles with government troops.
The fighters, who have been involved in an on-off rebellion since December last year, accuse President Francois Bozize of failing to honour a peace deal, the BBC reports.
France has called for an emergency meeting of the United Nations Security Council and officials warned that French nationals should stay at home.
Rebel spokesperson Nelson Ndjadder said they had shot down a government military helicopter and were heading for the presidential palace on Saturday.
They 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 UN official in southern Bangui says people are in a state of panic.
In January, the rebels joined a power-sharing government 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 peacekeepers are based.
The country has been hit by a series of rebellions since independence from France in 1960.