A powerful cleric said Friday that Iran will put British Embassy staff members on trial for fomenting postelection turmoil.

The move is expected to increase Iran’s isolation and alienate Western nations that have been trying to keep options open with Tehran despite its crackdown on protesters.

The announcement fueled calls in Europe for tougher action against Tehran. Britain is pressing for members of the European Union to pull their ambassadors out of Tehran to protest the staff members’ arrests last week.

The standoff is a test of how far Iran’s clerical rulers are willing to go to shore up their position at home after the protests – even if they risk wrecking possibilities for dialogue between Tehran and the West, a major goal of President Barack Obama that Tehran cautiously welcomed.

After quashing the street demonstrations, Iran’s leadership has been trying to erase any lingering doubts about the legitimacy of President Mahmoud Ahmadinejad by portraying the unrest as sparked by foreign meddling, not by public anger over the June 12 election, which the protesters said was fraudulent. Prosecuting the detained Iranian members of the British Embassy could help boost its case before the Iranian public.

At the same time, the arrests test the U.S. and Europe’s policy, which has been to avoid an overly harsh reaction to Iran’s postelection crisis.

So far, the EU has taken an incremental approach. On Friday, a day after issuing a public call for the staffers’ release, governments across the 27-nation bloc summoned Iran’s ambassadors to present the demand in person.

Swedish Foreign Minister Carl Bildt said the EU’s “escalatory approach to Iran was working.”

But French President Nicolas Sarkozy said his country backs Britain’s push for tougher action, “so that Iranian leaders will really understand that the path that they have chosen will be a dead end.”

British Foreign Secretary David Miliband said his country is “deeply concerned” about the personnel, who he said “have not engaged in any improper or illegal behavior.” He said he would speak with Iranian Foreign Minister Manouchehr Mottaki about the issue.

Word of the trials came from Ayatollah Ahmad Jannati, an ultra-conservative who is one of the most prominent figures in Iran’s clerical leadership and is close to the country’s supreme leader.

Jannati took a tough line in a sermon to thousands of worshippers attending Friday prayers at Tehran University, accusing Britain of being behind the protests.

London “designed a velvet revolution” to topple Iran’s Islamic government and the detained staffers confessed to their role, he told the crowd, where some chanted slogans against the U.S. and Israel.

“In these events, their embassy had a presence,” he said. “Some people were arrested. Well, inevitably, they will be put on trial.”