Ayatollah Ali Khamenei, Iran's supreme leader, has accused the country's pro-reform opposition activists of violating the law by insulting the late leader of the Islamic Republic and threatened to act against them.

Khamenei's televised warning on Sunday came after state television broadcast images of what it said were opposition workers tearing and trampling on a picture of Ayatollah Ruhollah Khomeini.

He also said the opposition had encouraged Iran's enemies to undermine the Islamic system.

Referring to a disputed June election which the opposition says was rigged in favour of Mahmoud Ahmadinejad, the Iranian president, Khamenei said: "The election is over. It was legal and they could not demonstrate their claim."

Earlier, the opposition expressed concern that the authorities were preparing to step up action against it after official media said pro-reform students had torn up a picture of Khomeini during a rally on Monday.

'Identifying culprits'

Khamenei said that such opposition rallies were illegal and he urged authorities to identify "those behind the insult to Imam Khomeini".

"We, as followers of Imam Khomeini, will not tolerate any shortcoming in identifying, trying and punishing those behind the insult and those who carried it out," Iran's Revolutionary Guards said in a statement.

Some reformist websites have suggested that Mir Hossein Mousavi, the opposition leader, may be arrested, six months after his election defeat by Ahmadinjad plunged Iran into turmoil.

But any detention of Mousavi, who advocated a return to Khomeini's fundamental values during his election campaign, may provoke new opposition demonstrations.

Meanwhile, Mousavi has been quoted in the past as condemning the incident relating to the picture.

Analysts say the internal crisis has further clouded prospects for any resolution of a long-running row with the West over Tehran's nuclear programme, which the United States suspects is aimed at making bombs. However, Tehran continues to deny any such claims.

Khomeini led the 1979 overthrow of the US-backed Shah and remains widely revered in Iran. He died in 1989 and was succeeded by Khamenei as supreme leader, Iran's highest authority under its government system of clerical rule.