The president's supporters say a "Yes" vote will reduce poverty
Niger's military is set to vote in a referendum on President Mamadou Tandja's bid to serve a third term.
The government says the soldiers are voting early so they can ensure the safety of the rest of the people, who will go to the polls on Tuesday.
Earlier, opposition groups reiterated they would boycott the vote, which they have described a coup d'etat.
Mr Tandja has recently dissolved parliament and the constitutional court to push through his referendum plan.
He says the people of Niger want him to stay in power, and his actions reflect their will.
But his efforts have caused widespread protests in Niger and sparked international condemnation.
Huge posters of Mr Tandja have been plastered throughout the capital, Niamey, and other main cities.
The BBC's Idy Baraou in Niamey says state media has been calling on voters to say "Yes" to changing the constitution so the 71-year-old president can stay in office.
Former army colonel, part of 1974 coup
First elected in 1999
First Niger leader to be re-elected - in 2004
Says he must stay in office to continue economic projects
Critics say the referendum is the same as a coup
The official campaign says a "Yes" will improve people's lives whereas a "No" vote means the country will remain stuck in poverty.
Campaigning officially ended on Sunday at midnight local time.
The move would allow him an initial three-year term, and then he would be able to run for re-election with no term limits.
The AFP news agency reported that opposition leader Mahamadou Issoufou launched a final appeal on Sunday for "mobilisation to cause the illegal referendum to fail".
Speaking for the blanket group Co-ordination of Democratic Forces for the Republic (CFDR), he said: "Our duty as citizens is to defend the current constitution."
Mr Tandja was first elected in 1999, and then again five years later.
He had previously promised to quit in December this year, a month after presidential elections are due to be held.
But the president's supporters argue he should be allowed a third term.
They point to his success in sealing deals with international energy firms over oil and uranium exploration.
And the president has argued that he needs more time to finish other large projects, such as a dam on the River Niger, which would increase electricity generation and provide irrigation to help local farmers.
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