The situation in the capital Bissau was uncertain on Monday after heavy gunfire was heard earlier near a government compound where President Umaro Sissoco Embalo had been chairing a cabinet meeting.
If a coup were confirmed in the former Portuguese colony, this would be the second in West Africa in as many weeks after the military in Burkina Faso deposed the president there on January 24.
"The latest information I have is positive given that the president is already at his palace, at his official residence," Portugal's Foreign Affairs Minister Augusto Santos Silva said in an interview with Portuguese broadcaster RTP.
"But we still don't know if the attack is over," he added.
A Facebook post on the account of Sissoco Embalo said calm had reurned to the capital.
Earlier, the African Union and the West African bloc ECOWAS both condemned the "attempted coup" unfolding in Bissau. The AU added that some government members were being detained and called on the military to release them. It gave no details.
Political instability has blighted Guinea-Bissau for decades, with nine coups or attempted coups since independence from Portugal in 1974.
The Portuguese embassy urged its citizens in Guinea-Bissau to stay at home.
United Nations Secretary-General Antonio Guterres is "deeply concerned" by the reports from Guinea-Bissau, a spokesman said.
A security source with contacts inside the compound said an unknown number of people had been hit by gunfire. A second source said two people were dead, but it was unclear who they were.
Members of government did not answer telephone calls from Reuters seeking verification of what was happening.
Sissoco Embalo had begun chairing the extraordinary cabinet meeting about 10am, entering the building with a heavy security detail, a diplomatic source said.
The cabinet meeting was being held to prepare for a forthcoming ECOWAS summit in response to last week's military takeover in Burkina Faso, the latest in a rash of coups across the region in the past 18 months.
