President Mamadou Tandja is surrounded by bodyguards Tuesday as he leaves the mayor's office in Niger's capital, Niamey, after casting his vote in a constitutional referendum that would keep him in power. (Rebecca Blackwell/Associated Press)

Niger's president pushed ahead Tuesday with a controversial referendum on a new constitution that would remove term limits and grant him another three years in office with increased powers.

Security forces on trucks with mounted machine-guns blocked the streets around the mayor's office in Niamey, where President Mamadou Tandja cast his ballot.

Opposition leaders called the move "illegal" and are boycotting the vote. International donors may respond by cutting aid to one of the world's poorest nations.

Ruler of the uranium-rich country since 1999, Tandja, 71, has twice won elections that were hailed as free and fair.

But in the last months of his final term Tandja, taking the same path as many African strongmen before him, broke his promise to step down when his term expires Dec. 22.

The current constitution states no president can run for office more than twice, and has a clause that term limits cannot be amended.

The new constitution has been heavily criticized because it was drafted by a five-member panel appointed by Tandja.

Among the new powers it would grant the president are the authority to name one third of a new 60-seat senate and the ability to appoint a media czar who can jail members of the press considered a threat.

Billboards call the vote an "exercise in sovereignty," but in a country where 70 per cent of adults are illiterate, some voters have no idea what's at stake.

With files from The Associated Press