A throng of so-called war veterans, a group of fiercely loyal and often violent supporters of Zimbabwean President Robert Mugabe, attacked a convoy carrying American and British diplomats Thursday. The assault was followed by a standoff of several hours before the diplomats were able to drive on. And it was the latest sign of how tense Zimbabwe is as Mugabe prepares to face a powerful rival in a runoff presidential election. The opposition and rights groups have accused Mugabe of orchestrating violence to ensure he wins. Mugabe has led Zimbabwe since independence from Britain and was once hailed as a liberator who promoted racial reconciliation and economic empowerment. But in recent decades, he has been accused of holding on to power through fraud and intimidation, stripping Zimbabweans of political rights and destroying the economy. Opposition leader Morgan Tsvangirai spent nine hours in police detention in southern Zimbabwe Wednesday evening, his party said. He resumed campaigning Thursday. Tsvangirai said in an interview Thursday he was campaigning in an environment "meant to frustrate the opposition. But we are inspired by the enthusiasm of people who we are meeting on the ground." In a message to Zimbabweans released by his party Thursday, Tsvangirai said his detention was "nothing compared to the hardships millions of Zimbabweans have had to endure." "Today I am saying to the nation that the rebuilding of our beautiful country must begin now," Tsvangirai said. "The time of intolerance and destruction must end. The time for peace and prosperity begins with each one of you voting" in the runoff. Tsvangirai beat Mugabe and two other candidates in the first round of presidential voting March 29, but did not win the 50 percent plus one vote necessary to avoid a runoff, according to official results. Tsvangirai's party won control of Parliament in legislative elections held at the same, putting Mugabe's ZANU-PF in the minority in Parliament for the first time since independence. In 2002, Tsvangirai came close to toppling Mugabe in presidential elections the opposition and observers say were rigged. Tsvangirai only returned to Zimbabwe in late May to campaign. He had left the country soon after the first round of voting, in March, and his party has said he was the target of a military assassination plot. He has survived at least three assassination attempts. Thursday, rights activists in Zimbabwe said that alleged Mugabe supporters had 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. Tsvangirai's party says at least 60 of its supporters have been killed since the first round of voting. Speaking in Washington, US State Department spokesman Sean McCormack said the Western diplomats' experience Thursday was indicative of the "repression and violence" Zimbabwe's government is willing to use against its own people. The US and Britain say their diplomats were attacked as they investigated political violence in Zimbabwe. US 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. In Zimbabwe, US Embassy spokesman Paul Engelstad told The Associated Press the attackers beat a Zimbabwean US Embassy staffer and slashed the tires of some cars in the convoy. US Ambassador James McGee, who was not with the convoy, told CNN that Zimbabwean police and military officers and so-called war veterans, Mugabe's fiercest and most fiercely loyal 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. Five Americans, four Britons and three Zimbabweans were in the three-car convoy, he said. Speaking later in Washington, McCormack said the diplomats were released after being detained and harassed, calling their experience "absolutely outrageous." McCormack said the US planned to raise the issue in the U.N. Security Council and directly with Zimbabwean diplomats attending a UN food conference in Rome. British Prime Minister Gordon Brown's spokesman Michael Ellam said British diplomats were safe after being freed. "The Foreign Office has summoned the Zimbabwean ambassador to explain the situation," Ellam told reporters. None of the British diplomats were harmed, Britain's Foreign Secretary David Miliband said. Police spokesman Wayne Bvudzijena denied security agents had threatened the diplomats, saying instead that police were trying to rescue them from a mob. "It's unfortunate when diplomats behave like criminals and distort information," Bvudzijena said. "It is a very sad situation." McGee said Zimbabwean officials had been informed of the trip, as required. Deputy Information Minister Bright Matonga said that while the US ambassador had submitted the necessary documents, the British government had not. Matonga accused the diplomats of handing out opposition materials. In mid-May, McGee had led a similar convoy that was briefly stopped at a police roadblock. At one point then, 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 hit McGee's shins, but he was not injured. At the United Nations Thursday, officials said Secretary-General Ban Ki-moon has gained Mugabe's permission to send his assistant secretary-general for political affairs to help the nation try to hold a free and fair runoff. Ban plans to send Haile Menkerios, a Harvard-educated diplomat and former Eritrean ambassador, to Zimbabwe within days, as soon as Menkerios obtains a visa.