Mamadou Tandja's plans have caused unrest at home and abroad
France has called on Niger to restore a democratic framework, after weeks of turmoil caused by the president's attempt to hold on to power.
President Mamadou Tandja won 92.5% of the vote in a referendum last week, allowing him to change the constitution and run for a third term in office.
The French foreign ministry said Paris would watch closely when Niger held its parliamentary election on 20 August.
Meanwhile, police confirmed they had arrested a leading opposition activist.
Officials indicated that Marou Amadou was arrested over recent comments he made criticising the referendum.
Mr Amadou was previously arrested in late June and accused of sedition, before being released days later.
Mr Tandja's move to stay in power has caused protests at home and prompted some donors to suspend aid.
But he says he needs to remain as president to see through various economic projects he has begun.
France, the former colonial power, has been among the critics of his recent actions.
Foreign ministry spokesman Romain Nadal said the forthcoming legislative election should be "transparent, fair and democratic".
"We call on President Tandja to resume dialogue with all political forces and to make all commitments necessary for Niger to return rapidly to a constitutional and democratic framework," he said.
The 71-year-old president, whose second term ends in December, dissolved both parliament and the constitutional court to push through the referendum.
His backers say he has improved living standards during 10 years in power and deserves to remain in office.
The president says he needs more time to complete multi-billion-dollar projects such as a uranium mine, an oil refinery and a dam on the River Niger.
But his critics portray him as a classic strongman determined to hold on to power so he can benefit financially from the projects he has started.
The European Union has already suspended an aid payment and warned of "serious consequences" for its co-operation with Niger if the president carries through his plans.