Four others will remain in custody while an investigation continues, said spokesman Hassan Qashqavi.

The Foreign Office in London confirmed that "some" staff had been released, but would not say how many.

British Foreign Secretary David Miliband on Sunday protested the arrests, calling them "harassment and intimidation of a kind which is quite unacceptable."

Iran's intelligence minister has blamed Western powers for stirring up protests over its disputed presidential election, singling out Britain and saying the British Embassy in Tehran "played a heavy role in the recent disturbances."

"The fact that Iran is stable, calm and secure, they're upset with this," Intelligence Minister Gholam-Hosein Mohseni Ejei told Iran's government-funded Press TV.

The Iranian government has long accused other countries, especially Britain, of "meddling" in its affairs, but has offered no proof any outside forces have been involved in the unrest since the election.

On Wednesday, Iranian authorities said they had arrested several foreign nationals, some with British passports, for allegedly helping foment the disturbances.

Miliband rejected the allegation on Sunday.

"The idea that the British Embassy is somehow behind the demonstrations and protests that have been taking place in Tehran in recent weeks is wholly without foundation," he said.