President Mamadou Tandja changed the constitution to stay in power
The junta which overthrew Niger's president and seized power last week has appointed a civilian to be prime minister until new elections are held.
It chose Mahamadou Danda, who served as communications minister in a previous administration, which itself took power in a coup in 1999.
The ousted President, Mamadou Tandja, is still being held by the military along with some of his ministers.
No date for the new polls has yet been announced by the coup leaders.
Niger's main opposition party has called for Mr Tandja to be tried for high treason because of his decision last year to scrap limits on the presidential term in office, which went against a ruling by the Supreme Court.
Mohamed Bazoum, vice president of PNDS Taraya, told the BBC that the decision was a criminal offence.
There has still been no official statement on what will happen to Mr Tandja, the BBC's Caspar Leighton reports from the capital, Niamey.
The Red Cross in Niger has told the BBC that they have been allowed to visit some of the detainees, but without specifying whom.
The organisation said it had had full cooperation from the military authorities and that it had not had to raise any concerns about the prisoners.
Army in charge
Opposition leaders like Mr Bazoum and some human rights groups have welcomed the coup, with the junta saying it will oversee the writing of a new constitution.
Population 14 million, 61% live on less than $1 a day
Huge reserves of uranium, Chinese firms digging for oil
History of coups, assassinations and on-off rebellion by nomadic Tuareg people in the north
Source: World Bank
While the crime of high treason carries the death sentence, Mr Bazoum said he did not want the former president to be executed.
The coup leaders, calling themselves the Supreme Council for the Restoration of Democracy, have promised to turn Niger into a democracy.
In a radio broadcast on Monday evening, they said putsch leader Salou Djibo would act as both the head of state and government, ruling by decree.
He will have the power to appoint and sack the government and the new prime minister.
The military have reportedly used tanks to surround the house of Gen Moumouni Boureima, the former head of the army who refused to back last Thursday's coup.
Niger has experienced long periods of military rule since independence from France in 1960.
But Mr Tandja's supporters argue that his decade in power has brought a measure of economic stability to the poor West African nation.
Under his tenure, the French energy firm Areva began work on the world's second-biggest uranium mine - ploughing an estimated $1.5bn (£970m) into the project.
China National Petroleum Corporation signed a $5bn deal in 2008 to pump oil within three years.