The announcement by Ayatollah Ahmad Jannati came a day after the European Union demanded Iran release the staffers, who were detained on June 27. Britain is pressing EU countries to pull their ambassadors out of Tehran in protest.
Jannati, a powerful hard-liner who is close to Iran's supreme leader, told worshippers during a Friday prayer sermon in Tehran that the detained staffers "made confessions."
"In these events, their embassy had a presence," he said, referring to the post-election turmoil. "Some people were arrested. Well, inevitably, they will be put on trial."
He did not say how many staffers will be tried or on what charges. Earlier Iranian officials said all but one of the eight embassy personnel arrested on June 27 had been released, but European Union officials said they believed more than one was still being held.
In London, a Foreign Office spokeswoman said of Jannati's comments that British officials are "very concerned about these reports and are investigating."
In Sweden, which currently holds the rotating EU presidency, Foreign Minister Carl Bildt said "it's not acceptable to file charges against the ones released or the ones still in custody," though he added that the report had not yet been confirmed.
Jannati does not hold a position in the government or judiciary, but is the head of the Guardian Council, a powerful body in Iran's ruling clerical hierarchy that stands above the elected government. He is also close to supreme leader Ayatollah Ali Khamenei.
Jannati told the thousands of worshippers that the British "had designed a velvet revolution ... In March, they said (in their Foreign Ministry) that street riots were possible during June elections. These are signs ... revealed by themselves."
He also said those involved in protests "need to repent and ask God to forgive them."
Protests erupted in Tehran and other cities after official results showed a landslide victory for incumbent Mahmoud Ahmadinejad in the June 12 presidential election over his pro-reform rival Mir Hossein Mousavi. The pro-reform camp said the results were fraudulent.
The protests were quashed in a tough crackdown, and Iran's police chief has said 20 "rioters" were killed during the unrest. During his sermon, Jannati said seven or eight members of the paramilitary Basij militia were also killed. Basijis took a leading role in putting down the protests, often clashing with demonstrators.
Iran's ruling clerics have called the elections "pure" and "healthy" following Khamenei's declaration that the results would stand.
Still, Mousavi appears driven to maintain his opposition and even to raise the stakes - though there have been no protests since Sunday. In a challenging statement on Wednesday, he said he considered the government illegitimate and demanded political prisoners be released.
Jannati took a tough line, indirectly accusing Mousavi of treason.
Though he did not name Mousavi directly, Jannati pointed out that Ayatollah Ruhollah Khomeini, leader of Iran's 1979 Islamic revolution, once said that "anyone disrupts unity has not only committed a sin but also has committed treason against the Islamic Republic and the system."
Jannati demanded that those involved in the protests "repent and ask God to forgive them."