Mauritanian soldiers have seized control of the state radio and television station and main routes in the capital Nouakchott, reports say.
Troops have also blocked off access to the presidential palace, witnesses say.
President Maaouiya Ould Sid Ahmed Taya is out of the country attending the funeral of Saudi King Fahd.
Rebel soldiers came close to toppling Taya in June 2003 before loyalist forces regained control. The government says it foiled two attempts in 2004.
"I heard a burst of gunfire near the presidency. I saw scared people running away. Civil servants have all left their offices," a witness told Reuters news agency.
An Agence France Presse journalist in the city says that military vehicles equipped with heavy weaponry and anti-aircraft guns have been deployed.
State media broadcasts have been cut.
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