The African Union has condemned a coup in Niger, where soldiers have captured President Mamadou Tandja and suspended the constitution after a gun battle.
AU chief Jean Ping said he was watching developments "with concern".
West African bloc Ecowas said it "roundly condemned" the takeover and had dispatched a mission to make its position clear to the coup plotters. But one opposition activist told the BBC the soldiers were "honest patriots" who were fighting tyranny.
Mr Tandja provoked a political crisis last August when he changed the constitution of the uranium-rich country to allow him to remain in power indefinitely. The Economic Community Of West African States (Ecowas), which suspended Niger after Mr Tandja's actions, said it had "zero tolerance" for any unconstitutional changes of government.
"We condemn the coup d'etat just as we condemn the constitutional coup d'etat by Tandja," Ecowas official Abdel Fatau Musa told the BBC's Network Africa programme.
He said the group had already sent a team to Niger and would maintain sanctions "until constitutional order is restored". The BBC's Idy Baraou in the capital, Niamey, said on the morning after the coup, people in the city were going to mosques and shops as normal.
He said there was not an obvious military presence on the streets, but heavy artillery had been deployed around the presidential palace.
In a televised address on Thursday evening, a spokesman for the plotters announced that the constitution had been suspended and all state institutions dissolved. The junta, which has called itself the Supreme Council for the Restoration of Democracy, imposed a curfew and closed the country's borders.
The plotters said their aim was to restore democracy and save the population from "poverty, deception and corruption". The move came after gunfights around the presidential palace. Soldiers captured Mr Tandja while he was chairing his weekly cabinet meeting, a government source told the BBC.
One of the coup leaders, Col Djibrilla Hima Hamadou, was also behind the last military takeover in 1999. The president was assassinated during that coup, but civilian rule was restored within a year.
One opposition activist, Mahamadou Karijo, whose Party for Democracy and Socialism has been bitterly opposed to Mr Tandja's rule, praised the soldiers for fighting tyranny.
"They behave like they say they are not interested in political leadership, they will fight to save the Nigerien people from any kind of tyranny," he told Network Africa. The government and opposition had been holding on-off talks since December to try to resolve the country's political crisis. Mr Tandja, a former army officer, was first voted into office in 1999 and was returned to power in an election in 2004.
Niger has experienced long periods of military rule since independence from France in 1960. It is one of the world's poorest countries, but Mr Tandja's supporters argue that his decade in power has brought a measure of economic stability.
Under his tenure, the French energy firm Areva has begun work on the world's second-biggest uranium mine - ploughing an estimated $1.5bn into the project. China National Petroleum Corporation signed a $5bn deal in 2008 to pump oil within three years.
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