"We are deeply disappointed that Iran has detained some of Britain's staff in Iran," Brown said at a press conference. "Some of them have now been released, but we must now see that the others are set free to resume their work.
"Iran's actions, first the expulsion of two diplomats, and now the arrest of a number of our locally engaged staff, is unacceptable, unjustified, and without foundation.
"And we with our international partners will continue to make this clear to the Iranian regime."
Britain confirmed Monday that five of the nine British embassy staff arrested in Tehran had been released but denounced the continuing detention of four people.
Iranian authorities had arrested the nine local staff over their alleged role in stoking the unrest which followed the disputed presidential election.
Iran, which has already confirmed the release of the five, has singled out Britain as the chief culprit in what Tehran sees as meddling from foreign countries in its internal affairs following the election.
The charge is denied by Britain.
Speaking alongside Brown at a Downing Street press conference, European Commission chief Jose Manuel Barroso said: "I would like to express my full solidarity with the United Kingdom.
"Intimidation and harassment are unacceptable and they will be met with a strong collective European response."
