Niger's new ruling junta, which has seized power from President Mamadou Tandja, named squadron leader Salou Djibo as its leader on Friday.
Dissident soldiers stormed the presidential palace on Thursday afternoon and arrested Tandja, 71, whose attempts to cling to power beyond his second term have been blamed for the coup, DPA reported.
The junta said that the heads of ministries will run the government for the moment.
Colonel Goukoye Abdul Karimou, a spokesman for the soldiers, informed the nation of the successful coup after fighting around the presidential palace.
"The Supreme Council for the Restoration of Democracy (CSRD) has decided to suspend the constitution of the sixth republic and to dissolve all the state institutions," he said on state television, reading from a statement in a small office crowded with uniformed men.
"We ask the population to keep calm and remain united around the ideals of the CSRD, which will be able to make Niger an example of democracy and good governance."
The whereabouts of Tandja, who was seized during a cabinet meeting along with some ministers, is unknown, although some reports suggest he is being held at an army barracks.
It was not clear if there were casualties during the fighting, but a Niamey resident who lives near the presidential palace told the German Press Agency dpa that fighting was fierce.
"There were a lot of explosions and heavy weaponry and shooting for hours," the resident, who wished to remain anonymous, said.
However, Niamey was calm on Friday morning and the tanks and vehicles ringing the presidential palace were the only sign of the military on the streets, the resident said.
Shops were open and people moving about, although the junta has imposed an overnight curfew from 6pm to 6am, starting Friday evening.
The president last year rode roughshod over parliament and the constitutional court to extend his rule and allow himself a chance at another term, raising tensions in the uranium-rich nation.
The resident said that people were generally happy about the coup and believed Tandja had brought it upon himself with his actions.
West African regional group ECOWAS suspended Niger after Tandja extended his term by three years, while the European Union and the United States also imposed sanctions.
ECOWAS chairman, Nigeria's acting President Goodluck Jonathan condemned the coup attempt. The bloc has been mediating talks between the opposition and the government, but has made little headway.
The chairman of the African Union commission, Jean Ping, added his voice to the condemnations.
"The Chairperson of the Commission ... condemns the seizure of power by force that took place in Niger," the AU said in a statement.
"He calls for the speedy return to constitutional order and affirms the readiness of the AU, in close collaboration with ECOWAS, to facilitate such a process."
The US said it was likely the coup was related to Tandja's attempts to cling to power.
"President Tandja has been trying to extend his mandate in office," State Department spokesman PJ Crowley said Thursday. "Obviously, that may well have been ... an act on his behalf that precipitated this act."
He added that while the US does not condone the use of violence, the coup highlighted the need for Niger to move forward and hold elections.
Tandja argued that he had to extend his term to oversee mining and energy deals he claimed would pull the former French colony out of poverty.
Several international companies have uranium mining operations in the West African nation.
The president, a former army officer, came to power in 1999 in the formerly coup-prone country, which has undergone long periods of military rule since it broke from France in 1960.