The Iranian government vented its anger at Britain last night, declaring that London deserved a "punch in the mouth" for its role as the "chief culprit" behind the mass protests sweeping the Islamic Republic.

The Supreme Leader Ayatollah Ali Khamenei further upped the ante at home, branding senior opposition members "enemies of God" who deserved to be executed. And in what human rights groups and diplomats described as another "ominous development", Iranian police told leading opposition members that they could no longer guarantee their safety outside their homes.

Among those to receive the warning was Mehdi Karroubi, who came fourth in June's disputed presidential election and whose car was smashed up by government supporters at the weekend. His son, Taghi, said his father was effectively under "house arrest".

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The office of Iran's Supreme Leader, who possesses ultimate authority in the country, said: "Those who are behind the current sedition in the country ... are mohareb [enemies of God] and the law is very clear about punishment of a mohareb." Under Iran's Islamic law the sentence for mohareb is execution. The influential parliamentary speaker Ali Larijani joined in the call for punitive action, exhorting: "Identify them, arrest them and firmly punish those who insulted religion."

Tens of thousands of supporters loyal to President Mahmoud Ahmadinejad rallied in the centre of Tehran yesterday, vowing to "sacrifice their lives" and "fight the enemies". In such an incendiary atmosphere, opposition figures say that the official threats from the government are tantamount to a call to violence. Mohammadreza Naqdi, the head of the Basiji militia who led the bloody post-election crackdown, has already been openly vowing retribution against "traitors".

There were reports of fresh clashes between demonstrators and security forces at a Tehran university and in the central city of Shiraz, although these could not be independently confirmed.

Meanwhile, the government continued its raids on the opposition. Security forces have already been accused of targeting family members. The nephew of the opposition leader Mirhossein Mousavi, who lost the disputed poll to Mr Ahmadinejad, was among eight killed on Sunday, the bloodiest of the past 10 days of protest.

Yesterday Mr Mousavi's brother-in-law, Shapour Kazemi, was detained, and Shirin Ebadi, the human rights lawyer who won the Nobel Peace Prize in 2003, said that her sister had been arrested in an attempt to silence her. "This arrest is illegal because my sister is a dentist, she is not in any way active in human rights or politics ... and she didn't participate in any protests," Ms Ebadi told French radio. "She is detained for the sake of me."

It remains unclear how many people are now in custody. The opposition website Jaras estimated that more than 900 people had been arrested in marches that began following the death of the senior dissident cleric, Grand Ayatollah Hossein Ali Montazeri on 19 December. The government puts the number between 300 and 400.

With the protests showing no sign of subsiding, the government fell back on its popular line of defence, blaming "Western hands".

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"Trying to overthrow the system will reach nowhere... designers of the unrest will soon pay the cost of their insolence," the Revolutionary Guards said in a statement. "The opposition... is backed by foreign enemies."

The British ambassador to Tehran was summoned to the foreign ministry to answer charges of interfering. Iranian anger had been fuelled by Foreign Secretary David Miliband's public condemnation of the crackdown, although Washington, Berlin and Paris have been similarly critical. Iran's Foreign Minister, Manouchehr Mottaki, warned: "Britain will receive a punch on the mouth if it does not stop its nonsense."

The tension between Iran and Western powers is only likely to increase as the end-of-year deadline looms for Tehran to accept a deal to send low-enriched uranium abroad to be converted, and thus prove it is not interested in developing a nuclear weapon. Washington has said it is already considering next steps if the deal fails.

Last night, there were unconfirmed reports that Iran was trying to import 1,350 tonnes of purified uranium ore from Kazakhstan in violation of UN sanctions already in place. A summary of an intelligence report, obtained by the Associated Press, said that Tehran was willing to pay $450m for the shipment.

Rising tension: 11 days of protest

*19 December Senior dissident cleric Grand Ayatollah Hossein Ali Montazeri, an architect of the 1979 Islamic revolution, dies late at night, aged 87

*21 December Tens of thousands attend his funeral in the holy city of Qom; reports of clashes between opposition supporters and security forces

*23 December More clashes reported in central cities of Isfahan and Najafabad

*26 December Baton-wielding riot police clash with protesters in Tehran

*27 December At least eight dead following anti-government protests, held on seventh day of mourning for Montazeri which coincides with the Shia religious holiday of Ashura. Mousavi's nephew among the dead and opposition says more than 900 people arrested in Tehran and Isfahan

*28 December US President Barack Obama condemns the Iran's "iron fist of brutality" as reports surface of 20 opposition figures being detained.

*29 December Tens of thousands of government supporters rally to demand opposition be punished. President Ahmadinejad condemns opposition demonstrations as a foreign-backed "nauseating masquerade". Tehran summons British ambassador and chastises London.