Mr Miliband said intimidation must not become 'the order of the day'
Foreign Secretary David Miliband has denounced the "intimidation" of UK and US diplomats detained in Zimbabwe.
He said it was a "serious incident" and said such intimidation must not become "the order of the day" before the presidential run-off vote on 27 June.
Mr Miliband said the British diplomats were unharmed and no violence had been directed against them.
The US said it was "completely unacceptable" its diplomats had been detained for hours by police.
In a statement, Mr Miliband said: "It gives us a window into the lives of ordinary Zimbabweans because this sort of intimidation is something that is suffered daily, especially by those who are working with opposition groups."
The diplomats were held for several hours, as they investigated political violence ahead of a presidential run-off vote between President Robert Mugabe and the main opposition leader Morgan Tsvangirai on 27 June.
Mr Miliband said the diplomats had been going about "the normal business of diplomacy" when they were detained.
The Foreign Office has summoned Zimbabwe's High Commissioner in London. Mr Miliband said he would be told the world was closely watching developments in his country.
The foreign secretary said it was very important the international community ensured "there is an election that allows the democratic will of the Zimbabwean people to be heard loud and to be heard clear."
He added: "Diplomats are protected by papers and by international contacts in a way that too many Zimbabwean citizens are not protected.
"We have to be concerned obviously about British staff, but we also have to be concerned that intimidation does not become the order of the day in the run-up to 27 June."
Meanwhile US ambassador James McGee said there had been an earlier attempt to force the convoy off the road when they refused to go to a police station. He also said a Zimbabwean driver had been beaten up.
A spokesman for the police, Wayne Bvudzijena, insisted their officers had been trying to rescue the diplomats from a dangerous mob.
Shadow foreign secretary William Hague said it was a "flagrant breach of Zimbabwe's obligations" to protect diplomatic staff in their country.
And Liberal Democrat leader Nick Clegg said: "This cowardly attack demonstrates both the desperation and thuggishness of the Mugabe regime."
Later Lib Dem deputy leader Vincent Cable told BBC One's Question Time President Mugabe should be stripped of his knighthood.
He said: "He's an extraordinarily vain man who wears his knighthood with pride and we should be stripping him of it - ok it's a symbol but it's an important symbol."
Mr Miliband, who was also on the programme, said he had been "horrified" to discover President Mugabe had a knighthood. But he added: "I don't want to play his game, what he wants is an argument about a knighthood. This is about much much more than a knighthood."
Human rights groups say there has been an escalating campaign of state-sponsored violence ahead of the presidential run-off vote.
Movement for Democratic Change leader Mr Tsvangirai was freed on Wednesday, after being detained by police for eight hours.
Official results from the 29 March first round of the presidential election gave victory to Mr Tsvangirai, but not the 50% of votes needed to avoid a second round.