Iran has detained eight local staff at the British embassy in Tehran on accusations of having a role in post-election riots, local reports said.
UK Foreign Secretary David Miliband demanded their release, saying the arrests were “quite unacceptable”.
Relations between the countries are strained after Tehran accused the UK of stoking unrest, which London denies.
Some 17 people are thought to have died in street protests after the disputed 12 June presidential poll.
Tehran has expelled two British diplomats in the past week, and the UK has responded with a similar measure.
The arrests were first reported by the semi-official Fars news agency.
“Eight local employees at the British embassy who had a considerable role in recent unrest were taken into custody,” Fars said, without giving a source.
UK Foreign Secretary David Miliband expressed “deep concern” over the arrest of local staff on Saturday.
“This is harassment and intimidation of a kind that is quite unacceptable,” he told reporters at an international conference in Corfu. “We want to see (them) released unharmed.”
He said the British government had made a strong protest and denied accusations that the UK was behind unrest in Iran.
Poll verdict
Meanwhile, Iran’s powerful Guardian Council was due to give its verdict on the result of the disputed presidential election, which handed President Mahmoud Ahmadinejad a decisive victory.
But the BBC’s Jeremy Bowen in Tehran says there is much politicking taking place behind the scenes, and that the five-day deadline for the Guardian Council to return its verdict may be extended.
Our correspondent says there is an attempt to form a committee – including the disappointed presidential candidates – to oversee the recount of 10% of the votes, a move which they are resisting.
Another parliamentary committee is holding discussions with the grand ayatollahs in an attempt from pro-Ahmadinejad forces to put on a show of unity, he adds.
But opposition leader Mir Hossein Mousavi has not backed away from his claim that the election result was fraudulent, and has refused to support the Guardian Council’s plan for a partial recount.
Mr Mousavi has been calling for a full re-run of the vote, but said on Saturday that he would accept a review by an independent body.
However the Guardian Council has already defended President Ahmadinejad’s re-election, saying on Friday that the presidential poll was the “healthiest” since the Iranian revolution in 1979.
BBC