Reports from the west African state of Mauritania say President Maaouiya Ould Taya has been forced from his palace in a coup by rebel soldiers.
Nouakchott is a city under siege
The two sides have been battling on the streets of the capital, Nouakchott, and government buildings are said to have been ransacked.
A BBC correspondent says there are reports that troops loyal to the president are converging on the city and the rebels are preparing to confront them.
The uprising is the most serious challenge so far to the rule of President Taya, who made enemies among the country's Islamists by forging close ties with Israel after taking power in a coup of his own in 1984.
I have decided to lock everyone inside, all members of my family, because we are too scared to go out
Tank and small arms fire was first heard in the early hours of Sunday around the presidential palace and at the airport near Nouakchott.
Later smoke was seen rising from the palace, thought to have been taken by the insurgents and there are reports of widespread looting.
Shooting was reported late into Sunday evening in parts of Nouakchott.
Hospital officials said civilians were among the victims of the fighting.
City in fear
The whereabouts of the president and his family are unclear.
His staff would only say that they were in a "safe place".
A government official said the authorities had the situation under control and reinforcements from outside the capital were believed to be on their way.
Ould Taya himself came to power in a coup
However the rebels are believed to have captured the state radio building at the second attempt after initially being repelled.
Mauritanian state media have stopped broadcasting
There are unconfirmed reports that the army chief-of-staff, Colonel Mohammed Lanine Ndiayane, was killed in fighting on Sunday.
One resident of the city spoke to the Associated Press news agency by telephone.
"I have decided to lock everyone inside, all members of my family, because we are too scared to go out," the man said, too afraid to give his name.
The rebels are still to issue any communique expressing their objectives and no clear leader has so far emerged.
Several reports suggest that members of the armed forces sympathetic to Iraq are involved in the insurgency.
Though officially an Islamic republic, Mauritanian authorities have cracked down on suspected Islamists and politicians with links to former Iraqi leader Saddam Hussein since the beginning of the war in Iraq.
Dozens of people have been detained and charged with plotting against the state.
Mauritania's strongly Muslim population are largely opposed to its close ties with Israel, say correspondents.
It is one of only three states in the Arab League to hold full diplomatic relations with Israel.
Technically, Mauritania has had a multi-party system since 1991.
But the ruling Democratic and Social Republican Party has remained in control, and the opposition boycotted the last presidential election in 1997.