UN Secretary-General Ban Ki-moon claims Zimbabwe opposition leader Morgan Tsvangirai's decision to withdraw from this week's run-off presidential election is "understandable."

Speaking after a meeting with members of the UN Security Council, he also said he strongly discouraged the government of President Robert Mugabe from holding the second-round election on Friday as scheduled.

He said: "I would strongly discourage the authorities from going ahead with the run-off on Friday. It will only deepen divisions within the country and produce a result that cannot be credible.

"I would like to take this moment to say how distressed I am by the events leading to the understandable decision of the opposition candidate Morgan Tsvangirai to withdraw from the run-off election scheduled for this Friday.

"There has been too much violence and too much intimidation. A vote held in these conditions would lack all legitimacy."

Earlier, Mr Tsvangirai took refuge in the Dutch Embassy after raids on his party's headquarters.

Armed police arrived at the offices of his Movement for Democratic Change party in Harare, detaining scores of people seeking shelter in the building from political violence.

A campaign of terror is being waged on those opposed to President Mugabe ahead of the presidential election run-off.

Mr Tsvangirai, who earlier said he was prepared to negotiate with the ruling Zanu-PF party when the violence had ceased, has abandoned Friday's race, saying he is no longer prepared to play Mr Mugabe's "game".

He said: "We are prepared to negotiate with Zanu-PF but of course it is important that certain principles are accepted before the negotiations take place. One of the preconditions is that this violence against the people must be stopped".

Mr Tsvangirai's decision to quit followed the blockade of a campaign rally by thousands of pro-government militants and came in the wake of the murders of up to 85 MDC supporters.

Meanwhile, tens of thousands of others have been forced to flee their homes as people are beaten and tortured. Women and young girls have been raped and children burned alive, according to reports.

In recent weeks, MDC general secretary Tendai Biti was arrested and charged with treason, which carries the death penalty, and Mr Tsvangirai himself was detained several times.

Foreign Office Minister Lord Malloch-Brown has said the UK will push world heads to declare that Mr Mugabe is no longer Zimbabwe's legitimate leader and will urge a broader package of sanctions against his supporters.

He said: "Our objectives are to get in every forum possible a recognition that today President Mugabe no longer remains the proper rightful leader of the country.

"He has no claim under his own constitution for the presidency...we do not accept the status quo, we do not expect the international community to accept the status quo. I think we can look with reasonable confidence to broad- based action to make sure there's a resolution of this situation."

Earlier, Gordon Brown called on the international community to refuse to recognise the result of the run-off in Zimbabwe's "fraudulent" presidential elections.

He said: "The regime has made it impossible to hold free and fair elections in Zimbabwe. State-sponsored terror and intimidation has put the opposition in an untenable position.

"The international community must send a powerful and united message that we will not recognise the fraudulent election-rigging and the violence and intimidation of a criminal and discredited cabal."

South Africa President Thabo Mbeki is thought to be pressing for further talks between the two parties, in the hope of encouraging a government of national unity.

Sanctions currently involve an asset freeze and travel ban for Mr Mugabe and members of his regime, but it is not clear what else could be done without heaping further misery on the beleaguered people.

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