Iran's government threatened harsh action Saturday if opposition supporters take to the streets again to demand a new election in open defiance of the country's supreme leader.

Web sites run by supporters of opposition leader Mir Hossein Mousavi called for street protests at 4 p.m. (7:30 a.m. EDT, 1130 GMT) but the candidate himself issued no public statement.

Mousavi did not attend a meeting of the country's top election authority but one of his sites said he planned to issue a statement "soon." It did not elaborate.

The decision to rally or stay off the streets will be crucial for the opposition movement and the unprecedented threat it poses to Iran's cleric-led government. Rallying could spark a bloody crackdown on Mousavi's supporters, or greatly weaken the government by forcing it back away from its threat of violence. If the opposition heeds the official warnings and cancels, it could give the government the opening it needs to suppress its greatest internal challenge since the 1979 Islamic Revolution. RELATED'Movement by the people, for the people'Security and Defense: Better the devil you know?'Every knock on the door would send me into a panic as I waited'

It remains unclear if Mousavi has the power to halt street demonstrations even if he orders his supporters to stay inside. The government appeared to be preparing for a confrontation.

Police and members of the pro-government Basij militia were out in force on the streets on Teheran, particularly in central commercial districts.

Teheran Province Police Chief Ahmad Reza Radan said "police forces will crack down on any gathering or protest rally which are being planned by some people."

English-language state TV said the country's highest national security body had ordered security forces to deal with the situation. It did not elaborate.

The government statements were the most explicit warnings yet of force against protesters who gathered in massive rallies last week to demand the government cancel and rerun elections that ended with a declaration of overwhelming victory for hardline President Mahmoud Ahmadinejad.

Mousavi says he won but Ahmadinejad stole the election through widespread fraud.

Supreme Leader Ayatollah Ali Khamenei sided firmly with Ahmadinejad Friday, saying the result reflected popular will and ordering opposition leadersto end street protests or be held responsible for any "bloodshed and chaos" to come.

The statement effectively closed the door to Mousavi's demand for a new election, ratcheting up the possibility of a confrontation.

Iran's Interior Ministry raised the possibility of directly punishing the 67-year-old former prime minister, saying he would "be held responsible for the consequences of any illegal gatherings." The ministry accused him of supporting protests that "have lead to the disruption of security and public order," State Security Council secretary, Abbas Mohtaj, said in a statement on the ministry's Web site.

Mousavi and reformist candidate Mahdi Karroubi did not attend a meeting with Iran's Guardian Council on Saturday, state TV said. The council, an unelected body of 12 clerics and Islamic law experts close to Khamenei, investigates voter fraud claims.

A conservative candidate, former Revolutionary Guard commander Mohsen Rezaei, did attend, state TV said.

The state's English-language channel said the Guardian Council had agreed to a recount of 10 percent of the votes, without providing details.

State TV also said the Association of Combatant Clerics, a reformist group linked to former President Mohammad Khatami, a Mousavi ally, had told supporters to stay off the streets. The report could not immediately be confirmed.

Since the June 12 election, Mousavi has become the figurehead for a broad collection of demonstrators - from the most liberal-leaning reformists to religious conservatives. Some could be prepared to take their protests to the limit, but many others have no interest in an all-out mutiny against the country's Islamic system.

A first sign of possible resistance to Khamenei's orders to cease came shortly after nightfall in Teheran Friday. Cries of "Death to the dictator!" and "Allahu akbar" - "God is great" - rang from rooftops in what's become a nightly ritual of opposition unity.

A spokesman for Mousavi said Friday the opposition leader was not under arrest but was not allowed to speak to journalists or stand at a microphone at rallies. Iranian filmmaker Mohsen Makhmalbaf told the AP from Paris it's even becoming difficult to reach people close to Mousavi. He said he has not heard from Mousavi's camp since Khamenei's address.

Iranian authorities have interfered with the Internet and mobile phone networks and placed strict limits on the ability of foreign media to cover recent events, banning reporting from the street and allowing only phone interviews and information from officials sources such as state TV.

British Prime Minister Gordon Brown and other European Union leaders have expressed dismay over the threat of a crackdow and both houses of the US Congress approved a resolution Friday condemning "the ongoing violence" by the Iranian government and its suppression of the Internet and cell phones.