* Shirin Ebadi calls for fresh elections with U.N. observers
* Demands release of 500 people she says have been arrested
* Calls it "premature" to speak of another revolution
GENEVA, June 18 (Reuters) - Iranian Nobel Peace Prize winner Shirin Ebadi called on Thursday for Tehran to declare the result of its disputed presidential election "null and void" and hold new elections under the supervision of the United Nations.
The acclaimed Iranian lawyer, speaking after a meeting with U.N. High Commissioner for Human Rights Navi Pillay, also called for the unconditional release of about 500 people whom she said had been arrested in the past week.
Iran's election has provoked the biggest and most violent protests since the 1979 Islamic revolution, rocking the world's fifth biggest oil exporter which is also embroiled in a dispute with the West over its nuclear programme.
"My request would be that in order that things calm down, these elections should be declared null and void and new elections should be organised under the supervision of international institutions," Ebadi told Reuters in an interview in Geneva, speaking through an interpreter.
Iran's most famous human rights lawyer, whose influence in the country is seen as limited, called for United Nations observers to scrutinise a fresh poll.
"I think if new elections are organised, but if there are no international observers, no matter what the outcome of these new elections would be, it could be protested and rejected by one or the other of the parties," she said.
Tens of thousands of supporters of defeated candidate Mirhossein Mousavi marched in Tehran on Thursday wearing black and carrying candles, to mourn those killed in mass protests against an election the defeated candidate says was rigged. Iran's top legislative body, the Guardian Council, invited Mousavi and the two other candidates beaten by President Mahmoud Ahmadinejad to discuss their complaints on Saturday.
"I think that the level of resistance of Mr. Mousavi will depend very much on the level of resistance of the people. As long as the people resist, then Mousavi can resist," Ebadi said.
Asked whether her country was headed for another revolution 30 years after the overthrow of the U.S.-backed shah after months of demonstrations, she replied: "I think it's premature to talk about that."
Last December the headquarters of her Human Rights Defenders Centre was closed down on the ground it did not have a legal permit for its activities, drawing criticism from the West.
She voiced concern on Thursday that two of her staff were arrested this week, naming them as attorney Abdulfattah Soltani and Reza Tajik who handled press relations. (Editing by Laura MacInnis and Jon Hemming)
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