Niger referendum critics plan to boycott, block vote

* Niger opposition wants to disrupt referendum

* Parties tell supporters to do "everything"

* President pushes ahead with plans

By Abdoulaye Massalatchi

NIAMEY, July 10 (Reuters) - Niger's main opposition parties on Friday called on their supporters to block and boycott a planned constitutional referendum that would extend the president's rule in the West African uranium-producer.

Despite opposition from politicians and courts at home, along with donors and regional political groups, President Mamadou Tandja is pressing ahead with plans for an Aug.4 vote aimed at giving him another three years in charge after his mandate ends.

The Front for the Defence of Democracy (FDD), an anti-referendum coalition of 20 political parties and civil society groups, said the president was guilty of "high treason".

"If Tandja insists on pushing his plans for a referendum, the FDD will call on all democrats to block the holding of the vote in any part of the country," the coalition said in a statement on Friday.

The statement did not give details on how supporters should block the vote from taking place.

The Nigerien Party for Democracy and Socialism (PNDS) also supported the idea of disrupting the referendum.

"We are calling for an active boycott of this referendum. We will do everything to block it, to ensure that it takes place in extremely difficult circumstances," a party spokesman told Le Republicain, a local independent newspaper.

Tens of thousands of people have taken to the streets of the capital, Niamey, in several anti-referendum demonstrations. Opposition members of the election commission (CENI) also resigned from the body in protest this week.

But the president of CENI said on Thursday their withdrawal would not stop the vote from taking place and Tandja's supporters spent the week travelling around the vast desert nation drumming up support for the plan.

Tandja says he needs more time in power to complete political reforms and large infrastructure projects that would otherwise not be completed before he is due to leave power later this year when his second term comes to an end.

The projects include a hydro-electric dam, a Chinese-funded oil refinery and a 1.2 billion euro ($1.67 billion) uranium project with France's state-owned nuclear energy group Areva (CEPFi.PA) that should see Niger become the world's No.2 uranium exporter.

However, critics of the plan, which include West Africa's regional body ECOWAS, the United Nations, the United States and former colonial power France, say it is a threat to democracy and have threatened to take action. (Writing by David Lewis; Editing by Sophie Hares)