* Niger's top court annuls referendum plans
* Court says president's decree was illegal
* President is seeking to extend time in power
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By Abdoulaye Massalatchi
NIAMEY, June 12 (Reuters) - Niger's top court on Friday annulled President Mamadou Tandja's plans to hold a constitutional referendum aimed at extending his rule in the uranium-producing West African country, saying it was illegal.
Tandja is due to step down when his second term in office ends later this year but had called for an Aug. 4 referendum which could hand him another three years running the nation that soon hopes to become the world's No. 2 uranium exporter.
"The constitutional court annuls the presidential decree of June 5 ... relating to a referendum on the constitution," the constitutional court said in a ruling.
The ruling, seen by Reuters, said Tandja's decree calling the referendum violated several articles of Niger's constitution.
The court had previously given an opinion on the president's plans, saying they were unlawful, but Friday's decision is legally binding.
There was no immediate reaction from the government to the court's decision.
His plans have sparked protests that turned violent and drawn criticism from foreign donors and regional political bodies, which said they were a step backwards and threatened sanctions against Niger.
Opposition leaders had called for demonstrations this weekend against the planned referendum. Some of the president's allies have also criticised Tandja's bid.
A team of officials set up to draft the new constitution had presented their initial proposals to a cabinet meeting this week although few details of the new document emerged.
The president says he needs the time to introduce a fully-presidential system of government that will give the president more power and end current blockages in governance.
He also says people want him to complete large infrastructure projects, including a hydro-electric dam, an oil refinery and French energy giant Areva's CEPFi.PA 1.2 billion euro ($1.70 billion) Imouraren uranium mine.
The United States, however, has strongly warned against the plan, saying it would be a set-back for democracy, while West Africa's ECOWAS regional political body has threatened to impose economic sanctions if Niger behaves undemocratically. (For full Reuters Africa coverage and to have your say on the top issues, visit: af.reuters.com/ ) (Writing by Nick Tattersall and David Lewis; Editing by Janet Lawrence)