TEHRAN — Iran's hard-line Guardian Council disqualified more than 1,000 presidential hopefuls on Sunday, narrowing a diverse field of candidates for next month's election to just six conservative contenders.

The surprise announcement all but guarantees that a conservative will take over the presidency from moderate Mohammad Khatami, whose attempts at reform have been stifled in the increasingly rigid political climate of recent years.

Iran's largest reformist party decried the disqualifications and threatened to boycott the June 17 election unless the decision was reversed by the Guardian Council, which answers directly to the country's supreme leader, Ayatollah Ali Khamenei.

In a similar move last year, the Guardian Council disqualified more than 2,500 reformist candidates from parliamentary elections. Voter turnout plunged, staunch conservatives won control of the legislature, and despair rose among Iranians seeking a more moderate government.

The effort to consolidate power in the hands of conservatives comes at a sensitive time for Iran's leaders, who are negotiating with the West over the nation's nuclear program.

Iran says its aims are to generate electricity, but the United States has accused Tehran of secretly working to build a nuclear bomb. At least one of the reformist candidates disqualified Sunday had urged Iran to make concessions in the talks.

Rajabali Mazrouei, a prominent member of the largest reform party, the Islamic Iran Participation Front, urged the council to reconsider the disqualifications.

"We are warning the Guardian Council that we will not participate in the election if it doesn't reverse its decision," Mazrouei told Associated Press. "Barring reform candidates means there will be no free or fair election."

Former President Hashemi Rafsanjani, who is ahead in polls and is widely considered the front-runner, was approved by the council. Rafsanjani is seen as a political chameleon, a shrewd negotiator who has been able to stay near the center of power by negotiating with different camps.

Other candidates given the go-ahead include former Revolutionary Guard commander Mohsen Rezai, former police chief Mohammed Baqer Qalibaf, Tehran Mayor Mahmoud Ahmadinejad and former TV and radio chief Ali Larijani. All four are former military commanders with close ties to Khamenei.

Former parliament speaker Mehdi Karrubi was also approved. Karrubi is a onetime hard-liner who migrated to the reformist camp, only to lose popularity by adopting a series of increasingly conservative stances.

Women were banned from the race, as they have been in the past.

Also excluded was the Islamic Iran Participation Front's candidate, Mostafa Moin, a former minister of science, research and technology. A popular figure among students, Moin had focused his campaign on the nation's restive university campuses.

Rumors that Moin would be disqualified had begun to swirl in recent days, and the party warned the government against barring reformists.

"If parts of society feel that the right of voting has been taken from them, or that a president has been determined for them beforehand, they will have no reason or excuse to go to ballot boxes," Moin's party said in a statement.

The list of approved candidates was revealed unexpectedly on state-run television's evening news broadcast. By law, the Guardian Council had until Tuesday to convey its decision to the Interior Ministry, which would have two days to make a public announcement.

Ali Shakouri Rad, a director of Moin's campaign, alleged that the Guardian Council acted illegally by announcing its decision through the state media. Until the announcement from the Interior Ministry, he said, "the published names are not official and lack legal correctness."

Khatami, who by law cannot seek a third term as president, swept into office in 1997 with heady promises of reform. Many Iranians hoped that he would be able to counterbalance an unelected, conservative judiciary, push reformist legislation through parliament and create a more open civil society.

But he proved powerless against the clerics who ultimately control Iran. Hopes of change withered as dozens of newspapers were shuttered; student activists were jailed, and legislation was blocked.

The next president will inherit tough policy problems at home, including widespread unemployment, struggles over civil liberties and a massive youth population that is increasingly disaffected. Internationally, Iran is under intense pressure to relinquish its nuclear program and is grappling with the presence of U.S. soldiers on its eastern and western borders, in Afghanistan and Iraq.