Other graduated measures  like a slowdown in issuing visas to Iranian officials seeking to visit Europe and a potential withdrawal of all European ambassadors  would be considered, the diplomat said. He said Iranian diplomats would be told that the arrest of the embassy employees and the threat of trials were considered a threat to all European Union diplomatic staff members in Iran.

Image Ayatollah Jannati spoke at Friday Prayer in Tehran. He said that the embassy employees had made confessions. Credit... Abedin Taherkenareh/European Pressphoto Agency

The Iranian authorities accused the employees of fomenting and orchestrating the protests that drew tens of thousands of Iranians into Tehran’s streets after the June 12 election. The demonstrations provoked a security crackdown that had largely ended the public protests but not the political ferment over the elections. The hard-line incumbent president, Mahmoud Ahmadinejad, has been officially declared the winner by a landslide, but his main opponent, Mir Hussein Moussavi, has vowed to continue his campaign to have the official result declared fraudulent.

Ayatollah Jannati, who is an ally of Iran’s supreme leader, Ayatollah Ali Khamenei, did not say how many of the detainees would be tried or on what charges, news reports said. But, in unofficial translations provided by news agencies, he said that the British Embassy had a “presence” in the postelection unrest and that “some people” had been arrested. It was “inevitable” that they would face trial, he said.

The Guardian Council is an influential panel of 12 clerics whose responsibilities include vetting elections. On Monday it certified Mr. Ahmadinejad’s victory, a step that emboldened hard-line officials to warn of a harsher crackdown if protests continued.

Britain has sought diplomatic help from the European Union, which is thought to hold more sway with Iran. But some European countries, led by Germany, have been reluctant to risk worsening ties with Iran, particularly at a time when European diplomats have been pressing it for concessions over its nuclear program. They have argued that a withdrawal of envoys, a step urged by Britain, would leave few diplomatic options if the crisis deteriorated further.

But there were signs that the threat of trials had stiffened resolve in other countries.

“Our solidarity with Britain is total,” said President Nicolas Sarkozy of France, adding that France favored tightening sanctions.

The Iranian authorities have frequently blamed foreigners for the turmoil, but they have singled out the British as instigators. They cited Britain’s covert role in past political upheavals, including the toppling in 1941 of Reza Shah Pahlavi, suspected of having pro-German sympathies during World War II, and the ouster in 1953 of Prime Minister Mohammed Mossadegh after his government nationalized Iran’s British-dominated oil industry.