Soldiers appear to have taken over parts of Bissau, the capital of Guinea-Bissau in West Africa.
Heavy gunfire has been heard and witnesses says soldiers are in control of main roads in the city.
Troops are also reported to have taken control of the national radio station and the ruling party's headquarters.
There are also reports of fighting near the residence of outgoing prime minister, Carlos Gomes.
The BBC says the whereabouts of Mr Gomes and interim President Raimundo Pereira are currently unknown.
Mr Gomes came first in an inconclusive presidential election last month, but failed to win outright.
The runner-up, former president Kumba Yala, has said he will boycott the runoff vote scheduled for 28 April.
Guinea-Bissau, a former Portuguese colony until 1974, has a history of coups.