Ambassador James McGee described the incident as extremely serious
Police in Zimbabwe detained US and UK diplomats for several hours as they investigated political violence there, US ambassador James McGee has said.
Mr McGee told the BBC there had earlier been a bid to force their convoy off the road in the town of Bindura when they refused to go to a police station.
He said their tyres were also slashed and a Zimbabwean driver beaten up.
The US and UK governments denounced the incident and demanded an explanation from the Zimbabwean authorities.
Earlier, the South African government said Zimbabwe's main opposition leader, Morgan Tsvangirai, had been released by the police on Wednesday after the intervention of President Thabo Mbeki.
A spokesman for Mr Mbeki, Mukoni Ratshitanga, told the BBC that Mr Tsvangirai's Movement for Democratic Change (MDC) had contacted the president "to inform him about the arrest".
"Immediately thereafter, President Mbeki contacted the Zimbabwean government and appealed to them to release Mr Tsvangirai," he said.
Mr McGee told the BBC that five US embassy officials, two local staff, and four officials from the UK High Commission had been in Bindura, 80km (50 miles) north of Harare, when they were stopped by police.
"I think that it gives us a window into the lives of ordinary Zimbabweans, because this sort of intimidation is something that is suffered daily
"Zimbabwe is now a lawless country," he said. "They are not following their own laws. They are not following international law."
"The government is trying to intimidate diplomats from travelling to the countryside to witness the violence they are perpetrating against their own citizens."
Last month, Mr McGee and diplomats from five other missions, including the UK, were briefly detained by Zimbabwean security forces.
The White House denounced Thursday's incident as "completely unacceptable".
"It's outrageous, and we are contacting the Zimbabwean authorities about the matter," said US National Security Council spokesman Gordon Johndroe.
The US envoy to the UN, Zalmay Khalilzad said he would raise the issue at the Security Council later on Thursday, and that he hoped it would express outrage about the actions of Zimbabwean police.
The UK government meanwhile said it had summoned Zimbabwe's ambassador in London to explain what happened.
Foreign Secretary David Miliband said the diplomats were unharmed and that no violence had been directed at them, but nevertheless described the incident as "serious".
"I think that 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," he said.
The BBC's Caroline Hawley in Johannesburg says the incident comes as human rights groups talk of an escalating campaign of state-sponsored violence ahead of the presidential run-off vote between President Robert Mugabe and Mr Tsvangirai at the end of the month.
The MDC has said at least 65 of its supporters have been killed since the election's first round in March.
Mr Tsvangirai resumed his campaign after spending eight hours in detention, but there are growing fears about the credibility of the election, she says.
The US Assistant Secretary of State for African Affairs, Jendayi Frazer, told the BBC that the "conditions in Zimbabwe are not yet present to have a free or fair election on 27 June".
"We have to step back and stop the fiction that somehow we are going to be prepared for a run-off, unless there is some sort of aggressive action taken not only by the South African government but also by Sadc (Southern African Development Community) as a whole," she said.
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