Soldiers have taken over parts of the capital of the West African state of Guinea-Bissau, reports say.
Heavy gunfire has been heard and soldiers are in control of main roads in the city, Bissau, according to eyewitnesses.
There are also reports of fighting near the residence of outgoing Prime Minister Carlos Gomes.
Mr Gomes came first in an inconclusive presidential election last month, but failed to win outright.
Troops are also reported to have taken control of the national radio station and the ruling party's headquarters.
The whereabouts of Mr Gomes and the interim President, Raimundo Pereira, are currently unknown, reports say.
The second-placed candidate in the March election, former President Kumba Yala, has said he will boycott the run-off vote scheduled for 29 April.
Mr Yala alleged that the election had been fraudulent.
The emergency vote was called after the death in January of President Malam Bacai Sanha, after a long illness.
Guinea-Bissau, an impoverished former Portuguese colony, has been plagued by a long series of coups since gaining independence in 1974.
It has recently become an important staging post for gangs smuggling drugs from Latin America to Europe.