TEHRAN - Iran has arrested several people who were preparing to disrupt rallies on February 11 marking the 1979 Islamic revolution, police said on Wednesday, in a clear warning to opposition supporters planning new protests.

Opposition leaders have called on supporters to take to the streets on Thursday, raising the risk of renewed violence eight months after a disputed election plunged Iran into crisis.

The authorities say the pro-reform opposition will face a firm response if it tries to hijack state-sponsored celebrations of the anniversary, when they say the Iranian nation in a display of unity will punch the faces of its Western enemies.

The hardline leadership is also facing increased international pressure after the Islamic state announced this week an expansion of nuclear work which the West suspects is aimed at making bombs, a charge Tehran denies.

U.S. President Barack Obama said on Tuesday the international community was moving fairly quickly toward imposing broader sanctions on Iran, a major oil producer.

Events will be held across Iran to mark the revolution anniversary but the main official gathering will be at Tehran's Azadi square, where President Mahmoud Ahmadinejad is expected to speak to a crowd which usually numbers tens of thousands.

Opposition websites say the day belongs to all Iranians and have urged supporters to attend the rally too.

We are closely watching the activities of the sedition movement and several people who were preparing to disrupt the February 11 rallies were arrested, Fars News Agency quoted police chief Esmail Ahmadi-Moghaddam as saying, giving no details.

Playing down possible unrest, the official IRNA news agency quoted him as saying: There will be no worries in this regard. We are fully prepared for holding a safe and glorious rally.

He said police, the Revolutionary Guards and the Basij Islamic militia were ready for any possible incident ... and they will let no one create insecurity.

EMBASSY PROTESTS

Prominent opposition figure Mehdi Karoubi, who came fourth in the June presidential election, plans to attend Thursday's rally in Tehran, his Sahamnews website said.

Karoubi's office ... invited people to take part in the February 11 rallies and follow up their legal demands, silently but as strongly as before, it said, suggesting they should avoid provoking pro-government forces by chanting radical slogans.

Some Iranians said they were apprehensive before the annual events marking the 31st anniversary of the revolution that toppled the U.S.-backed shah. Many people are worried, one young academic said in Tehran, declining to be named.

Eight people were killed in clashes between the security forces and opposition backers in December, in the most serious bloodshed since the aftermath of the vote, when dozens died.

The opposition is showing no sign of backing down, despite many arrests in a continuing crackdown by the authorities.

The opposition says the June poll was rigged to secure Ahmadinejad's re-election. Government officials have denied the charge and portrayed the protests that erupted after the vote as a foreign-backed bid to undermine the Islamic Republic.

After the June election, thousands of people protesting against the conduct of the vote were arrested. Most of them have since been freed, although more than 80 people have been jailed for up to 15 years, including several senior ex-officials.

In January, Iran hanged two people sentenced to death in post-vote trials and at least nine others are appealing such sentences. The West and rights groups condemned the executions, accusing Iran of seeking to intimidate the opposition.

The opposition Kaleme website said on Tuesday that 116 university professors in Tehran had sent a letter to Supreme Leader Ayatollah Ali Khamenei asking for help in putting an end to arrests of students and other academics.

Frightening and threatening university teachers and students will undoubtedly have a destructive impact on the country's scientific progress, the letter said.

(Writing by Fredrik Dahl; editing by Janet Lawrence)

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