Mutinous troops led by an army colonel captured Niger's President Mamadou Tandja after a gun battle on Thursday, and said they were suspending the constitution and dissolving all political institutions.
Tensions had been high in the west African uranium exporter since Tandja changed the constitution to extend his rule last year, a move that drew widespread criticism at home and led to international sanctions.
"We, the security and defence forces, have decided to take responsibility for putting an end to the tense political situation that you are already aware of," a spokesman for the military junta, which called itself the Supreme Council for the Restoration of Democracy (CSRD), said in a statement on state television late on Thursday evening.
The statement made no mention of Tandja, but military sources said he had been seized along with several ministers.
They said soldiers led by Colonel Adamou Harouna had stormed the presidential palace in a four-hour gun battle in the heart of the capital, Niamey.