General Verissimo Correia Seabre has declared himself president of Guinea-Bissau after seizing power in a coup.
The army chief of staff says he will remain leader until elections are organised in the West African state.
Soldiers have announced they are setting up a transitional government to include "all national political orientations".
They are holding President Kumba Yalla in detention, although they have promised to release him later.
A curfew was imposed on Sunday night in the capital, Bissau, with soldiers patrolling the streets, but the city is reported to be calm.
All government ministers have been ordered to report to a public building in the capital of the former Portuguese colony.
President Yalla dissolved the government last November, promising new elections but they have been postponed four times since.
The coup came at dawn, following an announcement on Saturday that the president would have to cancel them again.
Radio stations broadcast a message from the army saying it had seized power because President Yalla had failed to resolve the country's problems.
General Correia Seabre (right) pledged to hold fresh elections
"I am going to assume the presidency of the republic until there are elections," General Correia told Portuguese state television.
"As a person, I have nothing against Kumba Yalla," he said. "He can stay in Guinea-Bissau as well as leave the country."
The fate of Prime Minister Mario Pires, who is in the south of the country, was not clear with reports that the army was seeking to detain him.
Portugal condemned the coup and called on the army to allow
long-delayed elections to go forward in its former colony.
"The Portuguese Government deplores the military coup d'etat
that occurred today in Guinea-Bissau and calls on its perpetrators
to immediately re-establish constitutional legality... ," the foreign ministry said in a statement.
Guinea-Bissau is one of the poorest countries in the world and has been beset by food shortages and strikes.
It has never really recovered from an army revolt and then a war in the late 1990s, our correspondent says.
The secretary general of the United Nations said in June that the country was embarking on a downward course.
The Organisation of West African States recently said it was worried, not least by the silencing of political opponents of the president.