BBC UPDATE:
AU chief Jean Ping said he was watching developments "with concern" after a day of gun battles culminated in a takeover led by Colonel Salou Djibo.
West African bloc Ecowas "roundly condemned" the coup and dispatched a mission to talk to the plotters.
But one opposition activist told the BBC the soldiers were "honest patriots" who were fighting tyranny.
Heavy artillery
Mr Tandja provoked a political crisis last August when he changed the constitution of the uranium-rich country to allow him to remain in power indefinitely.
The Economic Community Of West African States (Ecowas), which suspended Niger after Mr Tandja's actions, said it had "zero tolerance" for any unconstitutional changes of government.
"We condemn the coup d'etat just as we condemn the constitutional coup d'etat by Tandja," Ecowas official Abdel Fatau Musa told the BBC
AU chief Jean Ping said he was watching developments "with concern" after a day of gun battles culminated in a takeover led by Colonel Salou Djibo.
West African bloc Ecowas "roundly condemned" the coup and dispatched a mission to talk to the plotters.
But one opposition activist told the BBC the soldiers were "honest patriots" who were fighting tyranny.
Heavy artillery
Mr Tandja provoked a political crisis last August when he changed the constitution of the uranium-rich country to allow him to remain in power indefinitely.
The Economic Community Of West African States (Ecowas), which suspended Niger after Mr Tandja's actions, said it had "zero tolerance" for any unconstitutional changes of government.
"We condemn the coup d'etat just as we condemn the constitutional coup d'etat by Tandja," Ecowas official Abdel Fatau Musa told the BBC