Call it pulpit diplomacy. While Western politicians agonise over how to rebuke Iran without undermining talks on its nuclear aspirations, hardline mullahs use their Friday sermons to bullhorn their defiance to the world.

Three Fridays ago Ayatollah Ali Khamenei, the Supreme Leader, demanded that the massive street protests end. Almost immediately his forces flooded the streets and began suppressing the demonstrations.

Last week Ayatollah Ahmad Khatami, another hardline cleric, told the thousands gathered at Tehran University that the “rioters” should be mercilessly punished and their leaders executed. Yesterday it was the turn of Ayatollah Ahmad Jannati, 83, chairman of the Guardian Council, the country’s highest legislative body, who said the British had planned a “velvet revolution” by inciting riots. He added that some of the nine embassy staff arrested last weekend would face trial for their part in the plot.

Which of the local embassy workers will be charged with what is unclear, but on Wednesday Iran’s semi-official Fars News Agency said that one of them “had a remarkable role during the recent unrest in managing it behind the scenes”. Another had been a “main element behind the riots” but had been freed because she had diplomatic immunity.

The claim that they had confessed was ominous. State television has shown detained demonstrators apparently recanting and blaming Western powers for inciting them. They almost certainly did so under duress.

Ayatollah Janatti’s sermon may have played well with his domestic audience but it apparently hardened resolve in Europe, where governments were shocked by the arrests. Like Britain’s, their embassies cannot function without Iranian advisers, translators, guards and other staff.

Last Sunday the EU pledged a “strong and collective” response to the arrests. On Thursday there was a wobble, with the more cautious governments, led by Germany and Italy — Iran’s biggest trading partners — rebuffing British proposals that all 19 member states with embassies in Tehran should temporarily withdraw their ambassadors in protest.

After yesterday’s sermon, however, a source close to the EU’s Swedish presidency insisted that “all options remain open”. EU governments summoned the various Iranian ambassadors and told them there could be no “business as usual” while British staff were held. EU countries also stopped issuing visas to Iranian officials.

Iran will also be high on the agenda of an Anglo-French summit on Tuesday, and British officials are pressing for a tough statement from next week’s G8 summit in Italy, which could draw China, the US and other countries into the general condemnation of Tehran’s behaviour.

Even with this diplomatic firepower at its disposal, Western nations must somehow find a way of punishing the regime for its brutal crackdown without scuppering negotiations to halt its nuclear programme.

But while Iran’s Ministry of Foreign Affairs seems keen to stop the confrontation worsening, the Intelligence and Interior ministries and other branches of the regime are not.

They want the turmoil of the past month blamed on foreign powers, not the regime’s transgressions. The British Embassy is an easy target, partly because Iran’s two other favourite enemies, the US and Israel, do not have missions in Tehran.