By Fredrik Dahl

TEHRAN, June 4 (Reuters) - The United States should ease its pressure on Iran and seek detente as it did with communist China in the 1970s if it wants to support democracy in the Islamic Republic, an Iranian opposition politician said.

Iran and the United States have not had diplomatic ties since shortly after the 1979 Islamic revolution. Washington is leading a Western drive to isolate Tehran over its nuclear work, while Iran brands the United States "the Great Satan".

Ebrahim Yazdi, leader of the banned Freedom Movement, called on Washington to change its policies towards Iran and suggested U.S. moves to reduce hostility would help remove any justification for Tehran in cracking down on critics at home.

"If the U.S. decides to normalise relations with Iran it would benefit the cause of democracy in Iran," said Yazdi, a foreign minister in Iran's first government after the revolution that overthrew the U.S.-backed shah.

"Because then the Iranian authorities don't have any excuse to suppress us or to accuse us of being agents of the foreigners," he told Reuters in an interview on Sunday.

Tehran says the United States is using intellectuals and others inside the country to undermine the government through a so-called "soft revolution". The country last month charged three Iranian-Americans in Iran with spying.

Rights groups often complain that Iran jails pro-reform writers, journalists and intellectuals without due legal process. Iran routinely dismisses accusations of rights violations and says it does not hold any political prisoners.

Iran's supreme leader, Ayatollah Ali Khamenei, on Monday said Iranians rejected Western-style democracy. Iranian officials say Tehran is ready to hold broad talks with Washington but only once it changes its hostile policies.

Yazdi, who was a close aide to the Islamic revolution's founding father Ayatollah Ruhollah Khomeini, is an important opposition voice in Iran but has no influence on state policy and limited popular support in the country.

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Yazdi suggested the U.S. administration of President George W. Bush, who branded Iran part of an "axis of evil" after he took office in 2001, could learn from President Richard Nixon and his shift in U.S. policy towards China 35 years ago.

"That is a very positive historic experience," Yazdi said. "It was very good for world peace."

Nixon's visit to China in 1972 has often been portrayed as a remarkable success that allowed a vehemently anti-communist U.S. president to repair ties with China, put pressure on the Soviet Union and help ease Washington's path out of the Vietnam War.

"If the United States wants to help they must try to ... come to some sort of agreement with the Iranian government."

One U.S. gesture could be to "do something" about five Iranians held by its forces in Iraq since January, he said. Iran calls them diplomats. Washington says they backed militants.

"I think if they do it, then the Iranian authorities would be forced to respond positively," said Yazdi.

Iranian and U.S. officials held their most high-profile talks in almost three decades in Baghdad on May 28 about ways to end violence in Iraq four years after the U.S.-led invasion.

"That is a good start," Yazdi said. "Now they are shaking hands, they say hello to each other ... it is historic."

But Washington has ruled out broader talks on trade and other incentives until Iran halts sensitive atom work which major powers suspect is aimed at making nuclear arms. Iran denies the charge and has refused to stop nuclear work.

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