Dissident soldiers stormed the presidential palace Thursday afternoon and arrested Tandja, 71, whose attempts to cling to power beyond his second term have been blamed for the coup.
The AU announced the suspension following a Friday meeting of the body's peace and security council in Addis Ababa and said it would help the nation return to constitutional order.
The capital Niamey was calm in the wake of the coup, a resident told DPA on condition of anonymity.
Shops were open and people were going about their business, despite a ring of tanks and military vehicles around the palace.
The junta, which Friday named squadron leader Salou Djibo as its leader, has called off a curfew and also re-opened the nation's borders.
Gunfire and the crunch of heavy weaponry around the presidential palace heralded the coup Thursday. Up to ten people are believed to have died in the battle, which lasted several hours.
Colonel Goukoye Abdul Karimou, a spokesman for the Supreme Council for the Restoration of Democracy (CSRD), then announced that the junta had taken charge.
Tandja, who was seized during a cabinet meeting along with some ministers, is reportedly being held at an army barracks.
International condemnation has poured in, but residents have generally welcomed the coup.
The president last year rode roughshod over parliament and the constitutional court to extend his rule by three years and allow himself a chance at another term, raising tensions in the uranium-rich nation.
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 consensus of international opinion is that Tandja's actions prompted the coup, and there now could be an opportunity to move forward and return to real democracy - provided the junta is prepared to relinquish power.
Little is known about Djibo and Karimou, but one of the other coup leaders is purported to be Djibril Hamidou, who was the spokesman for the 1999 junta that ousted Colonel Ibrahim Bare and led to the elections that brought Tandja to power.
The ousted president, a former army officer, came to power in 1999 in the coup-prone country, which has undergone long periods of military rule since it broke from France in 1960.
