TEHRAN, Iran – Iran's Supreme Leader Ayatollah Ali Khamenei warned the country's reformist opposition on Friday it would face a "harsh response" for confronting the Islamic establishment.

Khamenei's tough warning, in a nationally televised Friday prayer sermon, comes as security forces have hiked up pressure on the opposition's leaders, launching a series of raids that detained several of their top aides. In separate sermons, several hard-line clerics Friday repeated calls for the top opposition chiefs themselves to be arrested.

It was the second time that Khamenei has delivered Tehran's main sermon — a powerful political platform — since the disputed June 12 presidential election that sparked Iran's worst political turmoil in decades. The opposition claims President Mahmoud Ahmadinejad won the election by fraud and that the true winner was pro-reform candidate Mir Hossein Mousavi.

Hundreds of thousands of Iranians marched in mass protests in support of Mousavi, until Khamenei delivered a Friday prayer sermon soon after the vote backing Ahmadinejad's victory and demanding a halt to unrest. Security forces launched a crackdown that crushed the protests, arresting hundreds. The opposition says at least 72 people were killed, though authorities have confirmed only 36 deaths.

In his sermon Friday, Khamenei said peaceful differences and criticism of officials would be tolerated. But, he said, "confronting the system and drawing a sword against the system will bring a harsh response."

He also directly warned against protests on "Quds Day" — an annual event on Sept. 18, when the government organizes rallies in support of the Palestinians and against Israel. Some opposition activists have called for new anti-government protests on Quds Day — referring to the Arabic word for Jerusalem.

"The people should be on the lookout for some who use Quds Day to create divisions," Khamenei said.

Khamenei stands at the top of Iran's hierarchy of ruling clerics, and his word is considered supreme in political matters. The opposition's continued refusal to accept Ahmadinejad and its campaign against his government has been an unprecedented challenge to Khamenei's authority.

Hard-liners have accused the opposition of aiming to overthrow the clerical system with the wave of protests, and the government is holding a mass trial of more than 100 detained political activists and protesters who it claims provoked the mass demonstrations.

Ahmadinejad, hard-line clerics and commanders in the powerful Revolutionary Guard have called for the top opposition leaders to be arrested as well — referring to Mousavi and his allies Mahdi Karroubi and former President Mohammad Khatami. Detaining them would be a major escalation, a step that Khamenei so far has appeared hesitant to take.

Several prayer leaders around the country repeated those calls. In the southern city of Isfahan, Ayatollah Yousef Tabatabai asked why the judiciary has put lower-level opposition figures on trial "but does nothing with those who caused the recent events," the state news agency IRNA reported.

He said anyone calling for protests on Quds Day should be arrested.

In the past week, security forces launched a series of raids clamping down on an opposition campaign to investigate allegations that protesters arrested in the crackdown were tortured and raped — charges that have proved deeply embarrassing to the government. The raids shut down two offices connected to Karroubi and arrested four aides of Karroubi and Mousavi.