Col Goukoye Abdul Karimou read a statement signed by Col Salou Djibo
A coup has taken place in Niger and the president has been captured after a gun battle in the capital, Niamey.
In a televised announcement, a spokesman for the plotters said Niger's constitution had been suspended and all state institutions dissolved.
The junta imposed a curfew and closed the country's borders.
President Mamadou Tandja, in power for more than a decade in the uranium-rich nation, is believed to be in captivity at a military barracks.
Reports say government ministers are also being held.
Tensions have been growing in the country since last August, when Mr Tandja changed the constitution to allow him to stay in power beyond his legal term limit.
The move provoked a political crisis and threw Niger into isolation - regional grouping Ecowas (Economic Community Of West African States) suspending its membership.
A spokesman for the coup leaders said the country was now being led by a group called the Supreme Council for the Restoration of Democracy (CSRD).
He called on the people of Niger to "remain calm and stay united around the ideals postulated by the CSRD... [to] make Niger an example of democracy and good governance".
AT THE SCENE
Idy Baraou BBC News, Niamey
The exchange of gunfire has been between soldiers but it is confusing and one cannot tell one side from another. I saw tanks being fired and soldiers on the streets using machine guns.
The area near the presidential palace is where the business of government takes place and at least four military barracks are based there.
People have fled the area and some civil servants have locked themselves inside their offices.
Earlier, smoke could be seen from the roof of the office where President Mamadou Tandja was holding his cabinet meeting.
"We call on national and international opinions to support us in our patriotic action to save Niger and its population from poverty, deception and corruption," he added.
The BBC's Idy Baraou in Niamey says despite the curfew, people have gone to mosques and shops as normal.
He says there is not an obvious military presence on the streets, but heavy artillery has been deployed around the presidential palace.
The African Union has condemned the takeover saying coups were contrary to the AU's vision of a continent free of unconstitutional changes of government.
But one opposition leader, Mahamadou Karijo, welcomed the coup and praised the soldiers as "honest patriots".
"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 the BBC's Network Africa.
Soldiers captured Mr Tandja while he was chairing his weekly cabinet meeting, a government source told the BBC.
The government and opposition have been holding on-off talks since December - mediated by the regional body Ecowas - 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.