Opposition groups are urging voters to boycott the poll and the EU and UN have expressed concern at Mr Tandja's plans.

The BBC's Idy Baraou in the capital, Niamey, says the streets are quiet with only small groups of people voting.

He says security forces have fired tear gas at opposition supporters in their northern stronghold of Illela.

Some opposition supporters have also been arrested in Dosso in the east, after being accused of trying to disrupt the poll.

The president has dissolved both parliament and the constitutional court to push through the referendum.

His backers say he has boosted 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.

Our reporter says there is a heavy security presence in the capital, with troops and police on every street corner.

Markets are closed and the day has been declared a public holiday.

Polls are open until 1930 local time (1830 GMT).

EU warning

State media has been calling on voters to say "Yes" to changing the constitution so the 71-year-old president can stay in office.

The official campaign says a "Yes" will improve people's lives, whereas a "No" vote means the country will remain mired in poverty.

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.

Mr Tandja's government has meticulously organised the poll, with security forces voting on Monday to ensure they are free to guard polling stations on Tuesday.

Casting his vote in Niamey, Mr Tandja said he was "fully satisfied" that he had done his duty as president.

"It's a great day, our wish has been fulfilled," he said.

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.

Internal opposition has been led by Mahamadou Issoufou, who was beaten by Mr Tandja in presidential polls in 1999 and 2004.

He has been calling on the estimated six million registered voters to boycott the ballot.

There have been two huge rallies and two abortive general strikes by opponents of Mr Tandja.

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.