TEHRAN - Iran's top leader told the opposition on Friday they would face a harsh response if they drew their swords against the Islamic Republic, three months after a disputed election that sparked widespread unrest.

The stern warning from Supreme Leader Ayatollah Ali Khamenei, delivered at a Friday prayers sermon broadcast live by state media, was a clear signal he would not tolerate any perceived threat to Iran's clerical system of government.

It came at the end of a week in which three senior reformist figures, including allies of opposition leader Mirhossein Mousavi, were detained and the office of pro-reform cleric Mehdi Karoubi was closed, according to reformist websites.

Resisting the system and taking out the sword against the system will be followed by a harsh response, Khamenei said in his sermon at Tehran University, which was attended by hard-line President Mahmoud Ahmadinejad and top officials.

If somebody stands against the basis of the (Islamic) system and violates people's security, the system is forced to confront it, Khamenei said.

But Khamenei, who has final say on all matters of state, added criticism and differences among officials were acceptable.

It was the first time he led Friday prayers since a week after the disputed June poll, when he endorsed Ahmadinejad's re-election and accused Western powers of interfering in Iran's domestic affairs.

The election, which was followed by huge opposition protests, plunged Iran into its deepest internal crisis since the 1979 Islamic revolution.

It exposed deepening divisions within its ruling elites and added to tension with the West, already strained over Tehran's nuclear ambitions.

Khamenei did not address the dispute over Iran's nuclear program, two days after Tehran delivered proposals to world powers involved in efforts to resolve the issue diplomatically.

The West suspects Iran is seeking to build atomic bombs. Iran says it wants only electricity from uranium enrichment.

NO FAKE ISLAMIC REPUBLIC

Mousavi and Karoubi, who finished second and fourth respectively, say the poll was rigged to secure Ahmadinejad's re-election. Officials reject the charge.

The hard-line president shored up his position last week when parliament approved most of his new ministers, after almost three months of political turmoil in the major oil exporter.

But reformist leaders, including former President Mohammad Khatami, have made clear they will not give up their struggle, issuing several defiant statements over the last week.

Rights groups say thousands of people, including senior pro-reform figures and Khatami-era government officials, were arrested after the election. More than 200 remain in jail, according to the opposition.
Earlier on Friday, the website of Karoubi's party said a member of Mousavi's election staff had been detained, the third pro-reform opponent of Ahmadinejad to be held within a week.

Reformist websites reported the detentions of senior moderates Alireza Hosseini Beheshti and Morteza Alviri on Tuesday. Karoubi's office in Tehran was closed down the same day, and the premises of an opposition committee looking into the situation for detained protesters was raided.

Mousavi urged his supporters on Wednesday not to be provoked by the detentions, saying they were a sign of more horrendous events to come.

In his sermon, Khamenei said late revolutionary leader Ayatollah Ruhollah Khomeini in his time had dealt with people who had revolutionary and religious background and also held high positions, suggesting nobody was untouchable.

The system does not deal with anybody who has a different opinion as long as they move in the framework of principles and do not go after violence, he added.

He warned against any effort to turn Iran into a fake Islamic Republic ... with a secular nature but with an Islamic appearance as some had wanted in the past 10 years, in what appeared to be a reference to Khatami's 1997-2005 presidency.

(Writing by Fredrik Dahl; editing by Diana Abdallah)