Iran's supreme leader issued a tough warning Monday to the opposition to back down after the movement called for a referendum on the government's legitimacy, growing bolder in its challenge to the country's clerical rulers.

The opposition has been energized by a show of support last week from former President Akbar Hashemi Rafsanjani, a key figure within the clerical leadership. Far from ending as the supreme leader has demanded, the conflict appears to be moving to a higher level, to a struggle within the leadership itself.

On Monday, opposition leader Mir Hossein Mousavi made some of his harshest comments yet at hard-liners and, implicitly, the supreme leader himself. Mousavi said they had insulted Iran's people by claiming that the anger over June 12 presidential elections that exploded into massive protests was fueled by foreigners.

"You are facing something new: an awaken nation, a nation that has been born again and is here to defend its achievements," Mousavi said during a meeting with families of those arrested in the postelection crackdown. "Arrests ... won't put an end to this problem. End this game as soon as possible and return the sons of the nation to the nation."

Supreme Leader Ayatollah Ali Khamenei, who holds final say in all state matters in Iran, has declared valid the re-election of President Mahmoud Ahmadinejad and has demanded a stop to questioning the results. Mousavi and his camp claim Ahmadinejad's win was fraudulent and that the new government he is to form next month will be illegitimate.

Khamenei's warning on Monday was clearly aimed at telling the opposition to stay in line — and at Rafsanjani, hinting that the powerful cleric was not above punishment for his stances. The speech made clear the supreme leader was digging in against any talk of a referendum on Ahmadinejad's government.

Khamenei addressed his comments to "the elite," saying they could be careful in the positions they take on the postelection dispute and not do anything that will hurt Iran's security, which he said would be "the biggest vice."

"The elite should be watchful, since they have been faced with a big test. Failing the test will cause their collapse," Khamenei told a group of officials in a speech marking a religious holiday Monday, according to state radio.

"Anybody who drives the society toward insecurity and disorder is a hated person in the view of the Iranian nation, whoever he is," Khamenei said, as Ahmadinejad and other officials sat on the floor beside him on stage.

The opposition's persistence in its campaign against Ahmadinejad has become a direct challenge to the supreme leader, who is considered by conservatives as the representative of God's rule on earth, and have the final word deciding all political disputes.

Security forces crushed pro-Mousavi protests by hundreds of thousands of people in the weeks after the elections, arresting hundreds. At least 20 people were killed, though human rights groups believe the number is far higher. Hard-line clerics backing Khamenei have been telling Iranians to support the supreme leader and depicting the opposition as tools of foreign enemies like Britain and the United States.

Khamenei repeated that argument Monday, saying foreign countries' news media had fueled the protests by airing "procedures for rioting."

Opposition leader Mousavi responded sharply. While he didn't directly mention Khamenei, he appeared to address him — in a particularly defiant tone.

"Who believes that they (protesters) would conspire with foreigners and sell the interests of their own country? Has our country become so mean and degraded that you attribute the huge protest movement of the nation to foreigners? Isn't this an insult to our nation?" Mousavi told the families, according to pro-reform Web sites.

On Sunday, a leading reformist, former President Mohammad Khatami, called for a referendum to rule whether Ahmadinejad's government is legitimate and for a neutral body to oversee the vote.

The proposal appeared to be an attempt by the opposition to put forward a concrete demand around which supporters can rally to press on the leadership. Khamenei is unlikely to accept the idea, but just issuing the call is a sign of increased confidence among the opposition.

"I openly say now that the solution to get out of the current crisis is holding a referendum," Khatami said, according to reformist Web sites.

The Association of Combatant Clerics, a reform-minded clerical group to which Khatami belongs, echoed his call, saying in a statement that a referendum is needed because "millions of people have lost confidence in the electoral process."

Under Iran's constitution, a referendum has to be ordered by Khamenei himself. All popular votes in Iran are monitored by a powerful body of clerics called the Guardian Council. That council, which is handpicked by Khamenei and is dominated by hard-liners, oversaw the June 12 vote, then held a partial recount on orders of Khamenei to clear up the controversy. It ruled the results clean, but the opposition dismisses the recount, accusing the council of openly backing Ahmadinejad.

Khatami and the clerical group, however, proposed that "a neutral body" should monitor the proposed referendum instead. They proposed the Expediency Council, another clerical body that is headed by Rafsanjani.

"Of course, the referendum needs to be monitored not by bodies that brought this fate to the election but by a neutral body that people can trust," the association said, in a jab at the Guardian Council.

In his sermon Friday, Rafsanjani, also a former president, cast doubt on Ahmadinejad's victory and reprimanded the leadership for its harsh crackdown against peaceful protests and for not listening to the claims of fraud. He called for the release of all those detained as a first step toward a resolution.