Iran’s Supreme Leader Ayatollah Ali Khamenei today vowed he would not waver in response to protests over the disputed presidential election.

Claims of vote-rigging in the June 12 poll have provoked the biggest street demonstrations since the 1979 Islamic revolution and been answered by a bloody crackdown in which at least 18 people have died and more than 1,000 arrested.

“I had insisted and will insist on implementing the law on the election issue... Neither the establishment nor the nation will yield to pressure at any cost,” Ayatollah Khamenei said, effectively closing the door on compromise with the reformists.

Iran also stepped up its rhetoric against Western governments, whom Iran accuses of orchestrating the protests. This morning, Manouchehr Mottaki, the Iranian foreign minister, said that Tehran was considering whether to downgrade diplomatic ties with Britain.

Two British diplomats were expelled on Monday, accused of spying, prompting Gordon Brown to order tit-for-tat expulsions of two Iranian diplomats.

Mr Brown's spokesman said that Downing St was carefully considering Iran's latest move, and hoped to preserve a constructive relationship. “Iran’s decision to try to turn what are clearly internal matters for Iran into a conflict with the UK and others is deeply regrettable and without foundation."

It was reported today that one of Iran's most hard-line prosecutors has been appointed to lead the judicial crackdown on reformists. Human Rights Watch says that Saeed Mortazavi, the prosecutor-general of Tehran, has been put in charge of bringing dissidents' cases through a special court.

The New York-based campaign group says that Mr Mortazavi's credentials include personally interrogating Zahra Kazemi, an Iranian-Canadian photojournalist who died in custody in June 2003. Her family's lawyers say that her body showed signs of torture, including blows to the head.

The government of Canada continues to claim that not only did Mr Mortazavi order Kazemi's arrest, but supervised her torture and was present when she was killed. The Iranian Parliament has accused him of orchestrating a cover-up.

In 2000 while still a judge Mr Mortazavi reportedly ordered the closure of more than 100 newspapers and journals suspected of voicing dissent, and in 2004 he is said to have ordered the detention of more than 20 bloggers and journalists, who were allegedly held in solitary confinement in secret locations and coerced to sign false statements and make televised confessions.

Sarah Leah Whitson, HRW's Middle East and North Africa director, said: "Iran is in the midst of a violent and arbitrary crackdown on reformist protesters that has already claimed lives and led to over a thousand arrests.

"The role of Mortazavi in the crackdown suggests that the authorities are preparing to bring trumped up charges against its opponents."

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