AP photo, PA, Dominic Lipinski
Britain's Foreign Secretary David Milliband delivers a statement to journalists outside number one Carlton Gardens, London following the detention of United Kingdom and American diplomats by police in Zimbabwe, Thursday.

HARARE, Zimbabwe — U.S. and British diplomats were attacked Thursday as they tried to investigate political violence in Zimbabwe and a U.S. Embassy staffer was beaten, an embassy spokesman said.

U.S. officials in Washington and British officials in London said the diplomats were released after being held for hours at a road block on the outskirts of Harare, the capital, following a trip to northern Zimbabwe.

The opposition and rights groups have accused President Robert Mugabe of orchestrating violence and intimidation in the run-up to a June 27 presidential runoff.

Attackers beat a Zimbabwean U.S. Embassy staffer and slashed the tires of some cars in the convoy, U.S. Embassy spokesman Paul Engelstad told The Associated Press.

U.S. Ambassador James McGee, who was not with the convoy, told CNN that Zimbabwean police and military officers and so-called war veterans, a group of often violent Mugabe supporters, were responsible for what he called an "illegal action."

"The war veterans threatened to burn the vehicles with my people inside unless they got out of the vehicles and accompanied the police to a station nearby," McGee said, saying he was in touch with the group by mobile phone.

Five Americans, four Britons and three Zimbabweans were in the three-car convoy, he said.

Speaking later in Washington, U.S. State Department spokesman Sean McCormack called the detention of the U.S. diplomats "absolutely outrageous" and said it was indicative of the "repression and violence" Zimbabwe's government is willing to use against its own people.

McCormack said the U.S. plans to raise the issue with the U.N. Security Council and directly with Zimbabwean diplomats attending a U.N. food conference in Rome.

British Foreign Secretary David Miliband said none of the British diplomats was harmed.

"This is a window into the lives of ordinary Zimbabweans," Miliband said. "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" ahead of the presidential runoff.

Police spokesman Wayne Bvudzijena denied that security agents had threatened the diplomats, saying instead that police were trying to rescue them from a threatening mob.

"It's unfortunate when diplomats behave like criminals and distort information," Bvudzijena said. "It is a very sad situation."

In mid-May, McGee had led a similar convoy that was stopped at a police roadblock. Police eventually let the convoy through, and a patrol car escorted them back to the U.S. Embassy before disappearing.

At one point during the May incident, a police officer threatened to beat one of McGee's senior aides. The officer got into his car and lurched toward McGee after he had demanded the officer's name. The car made contact with McGee's shins, but he was not injured.

Meanwhile, Zimbabwe's opposition presidential candidate resumed campaigning Thursday, the morning after he spent nine hours in police detention near the country's second main city, his party said.

Morgan Tsvangirai said in a statement that the hours he spent in a Bulawayo police station after being stopped at a roadblock while campaigning demonstrate the lengths to which Robert Mugabe was prepared to go to "try and steal" the runoff.

But police spokesman Bvudzijena said police merely wanted to establish that one of the vehicles in Tsvangirai's convoy was properly registered. He said police had asked only the driver to accompany them from the roadblock to the station, but others in the party insisted on coming and waiting while the documents were reviewed.

Also Thursday, rights activists in Zimbabwe said that alleged Mugabe supporters petrol-bombed an office of Tsvangirai's Movement for Democratic Change in the southern province of Masvingo on Wednesday, killing at least two party officials.

At the United Nations, officials said Secretary-General Ban Ki-moon has gained Mugabe's permission to send a high-ranking U.N. envoy to help the nation try to hold a free and fair runoff.

Ban met with Mugabe on the sidelines of the U.N. food summit in Rome earlier this week and "highlighted the need to stop the violence and to deploy neutral international observers," U.N. deputy spokeswoman Marie Okabe told The Associated Press.

While talking with Mugabe, Ban suggested sending Haile Menkerios, a Harvard-educated diplomat and former Eritrean ambassador, to Zimbabwe "to discuss ways of how the United Nations can help in the election process," Okabe said.

Mugabe agreed to Ban's request, she told the AP.

Ban plans to send Menkerios, the U.N. assistant secretary-general for political affairs, to Zimbabwe within days, as soon as Menkerios obtains a visa.