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.

Five Americans, four Britons and three Zimbabweans were in the three-car convoy, U.S. Embassy spokesman Paul Engelstad said. The attackers reportedly slashed the tires of some cars in the convoy.

All were carrying out regular embassy duties and observing election run off procedures, a U.S. diplomat in Harare told FOX News.

A British official said all U.K. diplomats in Zimbabwe are safe following the incident, and the government has summoned Zimbabwe's ambassador to explain the attack.

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.

• U.S. Expresses Outrage Over Diplomats' Detention in Zimbabwe

He 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.

U.S. Ambassador James McGee, who was not with the convoy, said that Zimbabwean police and military officers and so-called war veterans, a group of fiercely loyal and often violent supporters of President Robert Mugabe, 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.

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

Police spokesman Wayne Bvudzijena denied 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.

Also Thursday, Zimbabwe's opposition presidential candidate resumed campaigning, 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.

The Associated Press contributed to this report.