Morgan Tsvangirai, the embattled Zimbabwe opposition leader, was arrested for the second time this week after police blocked him from reaching a campaign rally ahead of the critical presidential run-off in three weeks time.

Mr Tsvangirai’s detention came just two days after his last arrest and amid an escalating crisis surrounding the forthcoming polls as the government adopts ever more extreme measures to ensure Robert Mugabe’s victory.

International tensions reached a new high yesterday when British and American diplomats were detained, harassed and threatened by police and ruling party militia members as they investigated the increasingly brutal campaign of political violence.

Hours later, the government issued an edict banning the activities of all non-governmental organisations countrywide, cutting off vital food aid to more than four million of Zimbabwe’s neediest people and raising fears of an imminent humanitarian catastrophe.

Mr Tsvangirai was detained in Umzingwane after police set up a block across the road on the way to the rally site in Now Mine.

On Wednesday, police trapped him at a similar road block outside Bulawayo, releasing him eight hours later, only after the intervention of the South African President, Thabo Mbeki. Today he was released after two hours at a police station in Esigodini, 40 km (25 miles) southeast of Bulawayo, but ordered not to continue with his original plans.

Mr Tsvangirai’s party, the Movement for Democratic Change, said the harassment of diplomats and aid groups demonstrated how far the regime was prepared to go to cling onto power, and a forewarning of their lack of regard for the rule of law in the run-up to the election.

“The (MDC) president has just been released but instructed to go back to Bulawayo, instead of proceeding with the campaign. The police say the instruction came from the top,” MDC spokesman Nelson Chamisa said.

The party's secretary general, Tendai Biti, told the World Economic Forum for Africa in Cape Town: “It is almost as if the regime is sending out a message to the region, to the international community that it doesn’t care, that it has no respect for life, it has no respect for the rule of law. The regime is increasing the decibels of insanity.”

Mr Tsvangirai beat Mr Mugabe in the March 29 presidential poll, a shock defeat that triggered the current turmoil. Mr Tsvangirai claimed to have won the presidency with an outright majority but long-delayed and much discredited official results awarded him short of the number of votes required to avoid the run-off.

Since then, the Mugabe regime has launched an unrelenting campaign of violence against opposition members and supporters in which more than 2,500 have been wounded and more than 50 killed. Tens of thousands have been displaced, imperilling their chance to vote in the June 27 run-off.

But international condemnation reached a new decibel level today following yesterday’s edict forbidding all aid organisations from carrying out their work. Douglas Alexander, the secretary of state for international development, accused Mr Mugabe of “callous contempt” for the poorest and most vulnerable Zimbabweans.

“For Robert Mugabe to use the threat of hunger as a political weapon shows a callous contempt for human life,” he said. “This indefensible decision, which seems to be part of strategy ahead of the presidential run-off, demonstrated to the world the lengths to which Mugabe will go to cling to power.

In its edict, Zimbabwe’s government accused aid organisations of political involvement, including campaigning for the MDC, to justify the suspension. Aid organisations, however, have denied this, and charged that the government is simply trying to prevent them from witnessing state-sponsored violence and punish those who voted for the opposition by denying them access to aid.

Several hundred observers from the Southern African Development Commuity are due to arrive in Zimbabwe next week. But with larges swathes of the country now becoming no-go zones, it remains to be seen how effectively they can monitor the election campaign and voting.

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