Guinea-Bissau's military has taken control of the ruling party headquarters and national radio station on Thursday night as shots and rocket fire were heard in the capital, according to eyewitnesses speaking to Voice of America (VOA).
Reuters has also reported that soldiers surrounded the residence of former Prime Minister Carlos Gomes Junior, the frontrunner in the West African state's presidential election.
The apparent attempt at a coup comes the night before campaigning begins for a run-off presidential election in which the outgoing prime minister faces off against opposition candidate Kumba Yala.
Several political figures were reportedly "arrested" in the capital and taken to the army headquarters of Amura, near the coast, an anonymous military source revealed to AFP.
VOA reports that a Guinea-Bissau correspondent for Senegal's RFM radio said 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 told VOA he believes the troops may be affiliated with opposition presidential candidate Koumba Yala.
However, AFP reports that Gomes was among those "arrested" by the soldiers and "whisked away in a pick-up truck", according to the former prime minister's wife.
The military issued a terse statement on Friday saying the apparent coup was in response to a "secret deal" between Guinea-Bissau and Angola, both former Portuguese colonies, AFP reported.
However, the statement did not say who was now in control of the country.
The candidates were vying to replace the late president, Malam Bacai Sanha, who died in January after a long illness.
The opposition demanded the the cancellation of the presidential poll, charging that it was a sham, African Elections Project reported in March - "further stoking fears of trouble in the coup-prone state".
Guinea-Bissau has suffered through three coups and the assassination of a president since 1974.
Original article has been updated.