A group of military officers say they have seized control of 2 amid reports that the president, Umaro Sissoco Embaló, has been arrested.
Shortly after gunshots were heard in the capital, Bissau, government sources told the BBC that Embaló had been detained.
Military officers then appeared on state TV, saying they had suspended the electoral process and would be in charge until further notice.
The West African nation had been awaiting the outcome of Sunday’s presidential election, in which the main opposition candidate had been disqualified from running.
The results were expected on Thursday – both Embaló and his closest rival, Fernando Dias, have claimed victory.
Late on Wednesday afternoon, Embaló told France 24 in a phone call: “I have been deposed.”
Witnesses in Bissau heard gunfire earlier on, at around 13:00 GMT, but it was not immediately clear who was involved in the shooting or if there were any casualties.
Hundreds of people on foot and in vehicles fled, seeking shelter as the shots rang out, the AFP news agency reported.
Later on, General Denis N’Canha, head of the military household at the presidential palace, read out a statement declaring a takeover.
He said officers had formed “the High Military Command for the Restoration of Order” and closed the border.
Corbis via Getty ImagesGen N’Canha instructed the population to “remain calm”.
In addition to Embaló, the president’s army staff and a number of ministers have reportedly been arrested.
Guinea-Bissau, one of the poorest countries in the world, has seen nine coups or attempted coups since it gained independence from Portugal in 1974.
Embaló has said he has survived multiple coup attempts during his time in office. However, his critics allege he has fabricated crises in order to crack down on dissent.
Guinea-Bissau has a population of just under two million people and has been called a “narco-state” by the UN, due to its role as a key transit point in the trafficking of cocaine from Latin America to Europe.
