Uncertainty surrounded the fate of Guinea-Bissau's civilian leadership on Friday as a military coup appeared to be under way in the west African country.

Residents in the capital, Bissau, reported gunfire in the streets and heavily armed soldiers surrounding key locations.

"We heard gunshots in the street and now the military is surrounding the house of the prime minister, the headquarters of the ruling party and the airport," said Fadimata Alainchair, country director of NGO Plan International and resident in Bissau.

"The TV and radio are not broadcasting and no one knows what will happen next. We are just waiting for the announcement that will confirm the military have seized power."

The whereabouts of the prime minister, Carlos Gomes Junior – one of two contestants in a presidential run-off election that had been scheduled for 29 April – remained unknown after soldiers were seen hurling grenades at his home. The fate of the interim president, Raimundo Pereira, was also unconfirmed.

The apparent coup attempt follows weeks of speculation after elections held in the former Portuguese colony. The elections, which saw nine candidates vying for the presidency, were described as free and fair, but voters said they feared unease amongst the military would lead the result to be short-lived.

Gomes – seen as a frontrunner in the contest – attracted wide popular support but disdain from the army for his pledge to instigate military reform. Guinea-Bissau's army has been powerful since independence from Portugal, and is believed to be the main beneficiary of an estimated 800kg-1,000 kg of cocaine flown into the country every night.

Cocaine trafficking by air and sea – where Latin American criminals exploit the porous borders, uninhabited archipelagos and weak governance of Guinea-Bissau as a staging post to Europe – has led the country to be known internationally as a narco-state.

If confirmed, the attempted coup will be the latest in a long-running pattern of military takeovers, with no democratically elected leader in Guinea-Bissau ever having completed a term in office since independence was seized by the military from Portugal in 1974.

Last month's elections followed the death of President Malam Bacai Sanhá, who died in January from complications related to diabetes, but previous presidents were all toppled in the country's numerous military coups, including João Bernardo Vieira who was killed by soldiers in March 2009.

West African regional body Ecowas said it "formally condemns any attempt at a coup d'état," according to Daniel Kablan Duncan, president of the body's Council on Mediation and Security.