Thousands of protesters took to the streets of Niger's capital, Niamey, calling for the resignation of President Mamadou Tandja.
Brandishing placards such as "Tandja must go" and "Down with the Destroyer of democracy,", demonstrators on Sunday called for former prime minister and opposition figure Hama Amandou to take the president's place.
The opposition disputes an August 4 referendum that allowed Tandja to stay in power until 2012, after he was supposed to step down in December after two five-year terms in a row.
"It is up to us to end this autocratic rule," Muhammad Bazoum, a leader of the opposition Coordination of Democratic Forces for the Republic (CFDR), told the rally.
Tandja, 71, a former colonel in power for 10 years, also dissolved parliament and the constitutional court which had opposed the move.
The CFDR, which comprises political parties, human rights and labour organisations, has denounced the referendum as a "coup" and wants fresh elections to be organised.
The opposition boycotted October 20 legislative elections, after which the Economic Community of West African States (ECOWAS) suspended Niger as a member and the European Union put a freeze on its development aid.
ECOWAS mediator Abdulsalami Abubakar held talks with President Tandja on Friday as part of the 15-nation bloc's efforts to resolve the crisis, Nigerien state television reported.