Four people were killed in a failed coup attempt on Sao Tome, the state news agency STP-Press said on Sunday reporting a toll from the armed forces chief of staff.

The military, which on Friday thwarted a coup attempt in the tiny Portuguese-speaking archipelago considered a beacon of democracy, announced "four human lives were lost" after "exchanges of fire" at a military site.

Four men, including the former president of the outgoing National Assembly Delfim Neves, tried to attack army headquarters overnight, Prime Minister Patrice Trovoada said in a Friday video message confirmed by the justice minister.

He said Neves was one of several people arrested.

A resident speaking to AFP anonymously by phone said she had heard "automatic and heavy weapons fire, as well as explosions, for two hours inside the army headquarters" in the nation's capital.

In the video message, authenticated and sent to AFP by the press office of Sao Tome's prime minister, Trovoada is seen sitting at a desk saying he wants to "reassure" the population and "the international community".

Trovoada initially said a soldier had been "taken hostage" and wounded but "would be able to resume his activities in a few days".

A former Portuguese colony in the Gulf of Guinea, the nation of some 215,000 people is deeply poor and depends on international aid, but is also praised for its political stability and parliamentary democracy. (AFP)