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The opposition leader spent two hours on Monday being interrogated by police over the alleged treason plot - an offence punishable by death.

He said the police had told him he would be summoned to appear in court at a later unspecified date.

The MDC leader added that he believed that police would not proceed with a prosecution before the election.

US State Department spokesman Richard Boucher said he was not aware of any evidence to back the allegations and said it appeared to be "another tragic example of President Mugabe's increasingly authoritarian rule".

Australia threatened to impose sanctions and to ask the Commonwealth to withdraw election monitors from Zimbabwe if Mr Tsvangirai was arrested.

But Zimbabwe's Home Affairs Minister, John Nkomo, told the BBC the case was a matter of following the law.

"President Mugabe is not a person who came up with that videotape and President Mugabe did not set up that meeting," he said.

"Morgan Tsvangirai has admitted that he did indeed hold those meetings and as far as President Mugabe is concerned, he would be interested in knowing Morgan Tsvangirai's side of the story."

The BBC's Hilary Andersson says the affair's implications for Zimbabwe's elections are very serious, as the political temperature is already heated and many Zimbabweans will see this as an attempt by the government to ensure its key opponent does not win.

Secret filming

The allegations against Mr Tsvangirai were made by a Canadian political consultancy, Dickens and Madson, headed by former Israeli intelligence officer and Mugabe lobbyist Ari Ben-Menashe.

Mr Ben-Menashe says he was approached by Mr Tsvangirai, who wanted Mr Mugabe "eliminated".

A meeting with Mr Tsvangirai last December was filmed, but the MDC leader denies discussing the assassination of Mr Mugabe.

An independent media monitoring group, the Mass Media Project of Zimbabwe, said the videotape at the centre of the allegations had been heavily edited and even rearranged.

Last year, charges of treason against Mr Tsvangirai were dropped after a court ruled them unconstitutional.

Extract from the videotape:

Ari Ben-Menashe: "Who is going to call the suspension of the constitution and the emergency?"

Tsvangirai: "No they don't have to ... all they need to do is tell the Vice President 'Look we've got a crisis, we cannot proceed immediately after the head of state has been eliminated ... '

[gap]

Tsvangirai: "That is the fundamental issue ...

"That plus how the two forces, the MDC on the one hand and the army on the other, can work together to ensure a smooth transition towards democracy through an election process, even if it means delaying the election."

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