TEHRAN - Tens of thousands of government supporters rallied on Tuesday, state media said, and a reformist party called on Iran's rulers to apologise to the nation two days after eight people were killed in anti-government protests.

The rallies on Tuesday called for the punishment of opposition leaders for fomenting unrest after June's disputed presidential poll which was won by hardliner Mahmoud Ahmadinejad, state media reported.

The elite Revolutionary Guards accused the foreign media of joining hands with the opposition to harm the Islamic state and the British ambassador to Tehran was summoned by the Iranian government to be accused of interference in state matters.

If Britain does not stop talking nonsense it will get a slap in the mouth, Iranian Foreign Minister Manouchehr Mottaki said. The British government said their envoy would respond robustly to any criticism.

The establishment intensified a crackdown on the reform movement on Sunday by rounding up leading moderates to try to end street protests after the deadly weekend clashes erupted during the Shi'ite Muslim religious ritual of Ashura.

At least 20 opposition figures have been arrested since Sunday, including three senior advisers to opposition leader Mirhossein Mousavi, his brother-in-law and a sister of Iranian Nobel Peace Prize winner Shirin Ebadi, opposition websites reported. Ebadi confirmed her sister's arrest.

Political turmoil has entered a new phase in Iran with bloody face-offs and arrests, with security forces calling on authorities to deal firmly with opposition leaders.

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, which has joined hands with the foreign media, is backed by foreign enemies.

In a heated war of words, the reformist Islamic Iran's Participation Front said in a statement: The only way out of the current crisis is for the authorities to respect the law and apologise to the nation.

Tens of thousands of people on Tuesday chanted We are ready to sacrifice our lives for our Supreme Leader (Ayatollah Ali Khamenei), state television reported, saying the nationwide demonstrations had taken place spontaneously.

Demonstrators demand the punishment of those behind Sunday's protests which insulted religion, state television reported, referring to the protests taking place during Ashura.

The scale of the pro-government rallies could not be independently verified because of restrictions on the foreign media's movements because of anti-government protests.

Iranian authorities say eight people were killed in clashes on Sunday when supporters of Mousavi used the Ashura religious festival to stage fresh anti-government rallies.

Police said the suspicious deaths were being investigated, adding dozens of security men were injured in the clashes.

Authorities blame what they call foreign-backed terrorist groups for the killings, including the death of Mousavi's nephew Ali Habibi Mousavi Khamene.

What happened on Ashura day was an unsuccessful pre-planned scenario to harm the Islamic state's image and weaken the system, the semi-official Fars news agency quoted the head of the volunteer Basij militia, Mohammadreza Naqdi, as saying.

Parliamentary speaker Ali Larijani urged the judiciary to arrest those behind the anti-government rally on Sunday. Identify them, arrest them and firmly punish those who insulted religion, Larijani said, according to state television.
Larijani also called on opposition leaders to refrain from calling any more protests against the clerical establishment.

When the June 12 presidential election returned Ahmadinejad to power by a wide margin, thousands of Iranians took to the streets in the biggest anti-government demonstrations in the 30-year history of the Islamic Republic.

Street protests have shown no sign of abating since the vote, which authorities say was the healthiest since the 1979 Islamic revolution which toppled the shah. They strongly reject the opposition leaders' accusations of widespread vote fraud.

Opposition website Jaras said more than 900 protesters were arrested on Sunday in Tehran and the central city of Isfahan. Police said 300 people had been arrested in Tehran.

U.S. President Barack Obama strongly condemned on Monday what he said was the iron fist of brutality used to quell the protests and demanded the immediate release of detainees.

(Editing by Peter Millership)