Niger anti-referendum protests turn violent

* First violent protests against Tandja's power plan

* Widespread domestic opposition to referendum bid

NIAMEY, June 1 (Reuters) - Security forces in Niger fired teargas at people protesting on Monday against President Mamadou Tandja's plan to change the constitution to extend his rule of the uranium-mining state, officials and witnesses said.

Protesters set fire to vehicles in the town of Dosso, 140 km (88 miles) to the west of Niamey, in the first violent display of disapproval of Tandja's bid to hold a referendum on changing the constitution so he can stay in power another three years.

Opposition to the president's plans had so far been peaceful but investors have expressed fears that heightened political instability may delay mining projects in a country that hopes to become the world's No. 2 exporter of uranium.

In the latest sign of rising tensions, hundreds of youths opposed to the plan threw stones at a pro-Tandja meeting, regional governor Issoufou Oumarou said on local radio.

"For the moment we don't know if anyone has been killed or injured and there have been no arrests, but it's clear that those responsible for these acts will face the consequences."

Despite a 20,000-strong demonstration last month and a ruling from the constitutional court last week that a referendum was illegal, Tandja this weekend reaffirmed his intent to hold a plebiscite.

"The protesters ... smashed doors and windows, and also vehicles," said Dosso resident Gado Niandou. "They destroyed large hoardings showing picture of President Tandja, and in the streets they set fire to tyres."

Last week Tandja, whose second and final term in office is due to expire later this year, dissolved parliament saying he had done so to ensure stability.

Some 20 political parties and civil society groups have formed an anti-referendum coalition, while regional body ECOWAS has said neighbouring countries could punish Niger with sanctions if it behaves undemocratically. (Editing by Richard Balmforth)