Officials from the Movement for Democratic Change said their leader and a number of officials were stopped at a roadblock north of the second city, Bulawayo, yesterday morning and prevented from leaving. After a two-hour standoff, Mr Tsvangirai and his entourage were taken to a nearby police station and detained for several hours.

"They have not given us any reason for the arrest. The police just said our bosses want to see you," said Nelson Chamisa, the MDC's spokesman.

Mr Tsvangirai, who returned to Zimbabwe to campaign for the run-off election on 24 May, despite assassination fears, has twice faced treason charges which carry the death penalty. In March last year, he was beaten unconscious by police after attending an illegal prayer meeting.

The latest arrest is part of a harassment campaign against the MDC, which has left 58 of its members dead since the 29 March elections, which Mr Tsvangirai insists he won by the required 51 per cent majority.

"This is aimed at stopping him from campaigning and shows that this is not a free and fair election," said Carolyn Norris, the Zimbabwe expert for Human Rights Watch. The United States State Department said the arrest was "deeply disturbing" and called for his release "unharmed".

Since the release of the presidential election result on 2 May, in which the challenger came ahead of the Zimbabwean President and forced him into a humiliating run-off, Mr Mugabe's supporters have engaged in electoral cleansing by systematically targeting MDC voters in order to prevent them from casting their ballots where they are registered. The second round is scheduled for 27 June.

Sources inside Zimbabwe said yesterday that the army was now taking an active role in forcing people from their homes in areas of northern Zimbabwe, which had been strongholds of the ruling Zanu-PF party but where the MDC made significant inroads in the first round. "They see these people as traitors," one person said. "All control has been ceded to the army," whose leaders remain fiercely loyal to the President.

Human Rights Watch called on the government yesterday to reverse its ban on the distribution of urgently needed food aid by Care International, which was accused of campaigning for the opposition. The charity Save the Children also said yesterday that its workers had been ordered to suspend work in Binga district, where the agency was providing support for 60,000 children.

"The decision to let people go hungry is yet another attempt to use food as a political tool to intimidate voters ahead of an election," said Tiseke Kasambala, of Human Rights Watch.

The economic crisis, which has seen spiralling, six-figure inflation, mass unemployment and food shortages means many ordinary Zimbabweans, especially in rural areas, rely on charities for basic commodities. Mr Mugabe has regularly been accused of using food and distribution as a political tool, concentrating sparse stocks on ruling Zanu-PF areas or among card-carrying supporters.

Mr Tsvangirai's roadside detention came a day after the leader of the smaller MDC faction, Arthur Mutambara, was released from custody after his arrest on Sunday for writing an article critical of Mr Mugabe in Zimbabwe's only independent Sunday paper, The Standard. After his release, he accused Mr Mugabe of "violating the human rights of our people".

Cephas Zinhumwe, the chief executive of the National Association of Non-Governmental Organisations, said: "If we continue like this, we are going to have a crisis. The situation is ugly."

Mr Mugabe came under fierce international criticism this week after attending a UN summit in Rome on world food production while his people starve.

It also emerged yesterday that the acclaimed writer and journalist Peter Godwin was arrested 11 days ago by police for taking pictures in a church without permission. Mr Godwin said that he was taking photographs of his parents' graves but police suspected he was writing about a split in the Anglican Church in Zimbabwe, which has largely divided into pro- and anti-Mugabe factions.

Mr Godwin, whose books include Mukiwa: A White Boy in Africa and When a Crocodile Eats the Sun, chronicling the demise of Zimbabwe, was released the same day without charge.

From 'stolen' election to run-off

Held on 29 March 2008, the presidential results of Zimbabwe elections were not announced until 2 May.

Officials eventually proclaimed that the opposition leader, Morgan Tsvangirai, had won 47.9 per cent of the vote, with Robert Mugabe on 43.2 per cent. President Mugabe has been in power since 1980. However, the result has been challenged by the opposition Movement for Democratic Change (MDC), which says that election officials ignored its concerns over the vote counting. It believes Mr Tsvangirai won with 50.3 per cent of the votes.

The result means that a run-off, on 27 June, is required to produce a victor. In the build-up, a wave of political violence has engulfed areas where the ruling Zanu-PF did badly. According to the MDC, 25,000 people have been forced from their homes.

In the parliamentary elections, held at the same time, the MDC won 110 of the 210 seats. This means that, for the first time since Zimbabwe gained independence from Britain in 1980, the MDC has control of the legislature.

Last month, the MDC declared itself as the ruling party, and MPs met at the Harare International Conference Centre for what it called the first session of parliament.