Eyewitnesses say Guinea-Bissau's military has taken control of the ruling party headquarters and national radio station Thursday night.
The apparent coup attempt comes the night before campaigning was set to begin for a run-off presidential election pitting outgoing Prime Minister Carlos Gomes, junior against opposition candidate Kumba Yala.
Shots and rocket fire were heard in the capital Thursday night.
Soldiers also occupied central streets, including near Mr. Gomes home.
It is not clear if Mr. Gomes was at home. VOA?s Correspondent in West Africa, Anne Look, said her sources tell her Mr. Gomes? whereabouts are unknown.
But a Guinea-Bissau correspondent for Senegal?s RFM radio, reached by telephone late Thursday, told VOA there are unconfirmed reports the former prime minister and presidential candidate had been assassinated.
According to these reports, said RFM reporter Noah Mankali, ?Carlos Gomes, junior, was assassinated by the military, he is dead.?
Mankali said the soldiers apparently responsible for the coup attempt have so far not made any public statement. However he said he believes they may be affiliated with opposition presidential candidate Koumba Yala.
Mankali said the coup leader appears to be of the ?same ethnic [group] with Koumba Yala. ?I think that can be the origin of this coup d?etat [attempt],? he said.
In a news conference earlier Thursday, Yala warned against campaigning for the April 29 run-off election. He has previously called for a boycott of the vote, and said Thursday there would be "consequences" for anyone campaigning.
Yala was one of five candidates who claimed the first-round vote was rigged.
The candidates were vying to replace the late president, Malam Bacai Sanha, who died in January after a long illness.
Since winning independence from Portugal in 1974, Guinea-Bissau has struggled through a dictatorship, three coups and the 2009 assassination of a president.