Mr Tandja's critics accuse him of carrying out a coup
The opposition in Niger has condemned President Mamadou Tandja's decision to appoint a new constitutional court days after he dissolved the previous one.
Activists from unions, rights groups and opposition parties promised to hold protest rallies on Sunday.
Mr Tandja dismissed the country's top judges after they ruled that his plan to hold a referendum on extending his stay in power was illegal.
The US and UN have led criticism of Mr Tandja's attempts to stay in power.
In a statement, UN Secretary General Ban Ki-moon's office said the government's actions had "made it extremely difficult for the country's democratic institutions and the Constitutional Court to play their roles as guarantors of the rule of law".
And the EU said the president's actions raised "serious questions" about the amount of aid the bloc is prepared to give the country.
Mr Tandja also faces significant internal opposition - though calls for a general strike on Wednesday were ignored by many.
Under Nigerien law the president is allowed to serve only two terms, which would mean Mr Tandja would have to step down in December after 10 years in power.
But he claims that the people want him to serve a third term and has scheduled a referendum on the issue for 4 August.
Last week he announced he would rule by decree, because Niger's independence was under threat.
Parliament, which had opposed his plans, was dissolved in May, and earlier this week he dismissed the Constitutional Court.
The court ruled three times that he would need the support of parliament to hold a referendum.
The new court consists of three of the president's nominees, three magistrates chosen by the ministry of justice and a university professor.
Former Justice Minister Boube Oumarou will head the seven-strong court.
His supporters say Mr Tandja has brought economic growth to one of the world's poorest nations and so deserves the right to seek re-election.