David Miliband says he is "deeply concerned" about the arrest of the British embassy workers and has demanded their immediate release.
Reports say at least eight Iranian staff members were detained on charges of involvement in the country's political unrest.
The Foreign Secretary said he believed nine local staff had been arrested, but some have since been released.
He says the arrests are "harassment and intimidation of a kind which is quite unacceptable".
"At the moment our top priority is the position of our locally engaged staff, who we want to see released, unharmed and back at work," he said.
One has to ask how this situation has arisen. Has Iran threatened Britain in any way? Has it threatened anyone else? Nope, not as far as I'm aware.
This diplomatic debacle has come about thanks to the desire of some to preach about other country's internal affairs.
Let's look at a couple of examples. First the G8...
We deplore post-electoral violence which led to the loss of lives of Iranian civilians and urge Iran to respect fundamental human rights including freedom of expression and ensured by the international treaties it has ratified.
And now Gordon Brown using slightly subtler language to get the same point across...
We want a very good relationship with the Iranians, we also respect the fact that it's for the Iranian people themselves to choose who their government is.
But when there is a sign of repression or where there is violence that's affecting ordinary people in the streets, we have a duty to speak out and to say we want Iran to be part of the world, we don't want Iran to be isolated from the world.
The question here is, why bother? Nothing can be achieved by this sort "holier than thou" rhetoric. Ultimately it will be Iranians who resolve this crisis -- no-one else.
Also it's debatable whether Mousavi -- 'the good guy' -- would be much of an improvement for Iran. So again -- why bother?
All we've done here is heighten tensions between ourselves and Iran. And what good does that do? Absolutely none.
I've said it before and I'll say it again. We must return to a sensible, non-interventionist foreign policy because anything else just causes trouble.
And if you find that view objectionable I ask you to consider these fine words from Ron Paul...
I rise in reluctant opposition to H Res 560, which condemns the Iranian government for its recent actions during the unrest in that country. While I never condone violence, much less the violence that governments are only too willing to mete out to their own citizens, I am always very cautious about “condemning” the actions of governments overseas. As an elected member of the United States House of Representatives, I have always questioned our constitutional authority to sit in judgment of the actions of foreign governments of which we are not representatives. I have always hesitated when my colleagues rush to pronounce final judgment on events thousands of miles away about which we know very little. And we know very little beyond limited press reports about what is happening in Iran.
I adhere to the foreign policy of our Founders, who advised that we not interfere in the internal affairs of countries overseas. I believe that is the best policy for the United States, for our national security and for our prosperity. I urge my colleagues to reject this and all similar meddling resolutions.