UPDATE 1-Niger to hold constitutional referendum on Aug 4

(Adds quote, background)

NIAMEY, June 5 (Reuters) - Niger will hold a referendum on its constitution on Aug. 4, state television said on Friday, confirming President Mamadou Tandja's plans to shrug off criticism of his bid to extend his rule of the uranium-exporter.

Tandja is due to stand down when his second term in power runs out later this year but the president wants voters to approve constitutional changes that would give him three more years in power and hand future presidents more authority.

The plan has led to demonstrations in Niger, drawn criticism from donors and worried some investors who fear it will fuel political instability in the nation that hopes to become the world's No. 2 uranium exporter.

"According to a decree approved at a cabinet meeting, Nigerien voters will be called to vote in a constitutional referendum on August 4, 2009," state TV reported in a news bulletin on Friday evening.

Tandja is seeking another three years in power to introduce a fully presidential system of government that would give the president more power than he has under the current system, which gives some authority to a prime minister.

The president also says there is popular demand for him to extend his time in power so he can oversee the completion of large projects, such as the Kandadji dam, a Chinese-backed oil refinery and Areva's CEPFi.PA Imouraren uranium mine.

Donors have criticised the project and West African body ECOWAS last month threatened to impose sanctions if Niger undermined progress towards democracy.

A demonstration against the plan turned violent earlier this week but opponents are vowing to protest again on Sunday, despite a government ban against them doing so. (Reporting by Abdoulaye Massalatchi; Writing by David Lewis; Editing by Angus MacSwan)