Morgan Tsvangirai, the Zimbabwean opposition leader hoping to displace Robert Mugabe, has been detained by police again today while attempting to campaign for this month's presidential run-off vote.

Mr Tsvangirai and his campaign convoy were stopped at a roadblock and ordered to proceed to a police station in the town of Esigodini, near Bulawayo, Zimbabwe's second city. Reporters with the convoy said that police told him his planned rallies in the area were illegal.

The Movement for Democratic Change (MDC) leader was detained for nearly nine hours on Wednesday before being released without charge - prompting accusations that police loyal to Mr Mugabe were simply trying to stop him campaigning.

Yesterday a group of nine British and American diplomats were detained and threatened for several hours at a roadblock by Mugabe militiamen – plunging relations with the West to a fresh low.

Meanwhile, aid groups trying to operate in Zimbabwe were told that they would have to promise not to interfere in domestic politics if they want to get round a blanket ban imposed by the Government.

The ban was announced in a letter sent yesterday to non-governmental organisations by Nicholas Goche, the Public Service Minister, who ordered voluntary groups to "suspend all field operations immediately". Mr Goche said in that letter that had come to his attention that "a number of NGOs involved in humanitarian operations are breaching the terms and conditions of their registration".

The move was seen as part of a wider crackdown by Zimbabwean authorities ahead of a presidential run-off vote on June 27.

The New York-based group Human Rights Watch has accused the Zimbabwean Government of using food aid as a political weapon ahead of the presidential vote, an accusation repeated by Douglas Alexander, Britain's International Development Secretary.

Some 10 million Zimbabweans live below the poverty line and four million, particularly in rural areas, rely on food aid for basic commodities such as cooking oil and cornmeal.

But a government spokesman said today that NGOs had brought the ban on themselves by acting "like political parties".

"As we speak there are no NGOs. All NGOs have been asked to reapply for registration," said Bright Matonga, the Deputy Information Minister. "They were involved in political activities and behaving like political parties when they were supposed to complement government efforts.

"As it appears that they veered from their normal work, we want them to clearly state what they intend to do, so that they will be bound by that."

British charities gave warning last night that the ban on aid agency field work would be "extremely detrimental", especially for work supporting vulnerable children.

Oxfam said access for humanitarian relief agencies to Zimbabwe’s rural areas had been extremely limited. The charity said in a statement: "International NGOs - Oxfam included - have been unable to conduct food needs assessments, making preparation difficult for the impending hungry season.

"We reject the present government’s assertion that international civil society agencies are aiding and abetting the opposition."

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