Robert Mugabe was desperately attempting to cling to power tonight despite his clear defeat in Zimbabwe's presidential election by blocking the electoral commission from releasing official results and threatening to treat an opposition claim of victory as a coup.

The Movement for Democratic Change said that what it regards as the overwhelming win by its candidate for president, Morgan Tsvangirai, is "under threat" despite growing support from foreign monitors for its claim of victory.

The party also said it had "security concerns" after a police raid on its election offices today, and Tsvangirai made no public appearances apparently out of concern for his safety.

Mugabe's spokesman, George Charamba, warned Tsvangirai not to declare himself president because that "is called a coup d'etat and we all know how coups are handled".

But sources close to the MDC said that the party leadership has put out feelers to the military and elements of the ruling Zanu-PF to try and arrange a peaceful transfer of power.

Independent monitoring groups said that returns posted at about two-thirds of polling stations gave Tsvangirai 55% of the vote to Mugabe's 36%. The monitors said there is no way now for the president, who even lost in his home territory of Mashonaland as well as other former strongholds, to legitimately win the election.

A third presidential candidate, Simba Makoni, a former finance minister who broke with Mugabe, took about 9%.

Zanu-PF also suffered losses in the parliamentary election with at least nine members of its politburo losing their seats including the vice president, Joice Mujuru, and the defence, information and education ministers.

The MDC's secretary general, Tendai Biti, said the party was increasingly alarmed at the refusal of the state-run Zimbabwe Electoral Commission to issue any results.

"We are very much concerned by the lack of results from the ZEC. It appears the regime is at a loss how to respond and is therefore taking its time. We are really concerned by this assault on democracy. The primary point of an election is a result. We think there is a constitutional threat to those results," he said.

The commission has in the past begun issuing results as soon as they are posted at polling stations, and collated them by constituency for release within hours of the vote.

ZEC's chairman, George Chiweshe, declined to explain why he was still not issuing results more than 24 hours after the polls closed. "This is a complicated election and we will release the results when we have them," he said.

Opposition supporters in some towns, including Bulawayo, Mutare and Masvingo, publicly celebrated but generally Zimbabweans were cautious, not quite believing that Mugabe will leave office after 28 years in power.

With more than 50% of the vote, Tsvangirai would avoid a run-off election although his proportion might yet fall below the threshold as many of the remaining results are from rural areas where Mugabe traditionally has support.

Biti warned that there was still scope for fraud. He said his party is still encountering irregularities including the sudden appearance of additional ballot boxes at polling stations where the count has been completed. He also said that MDC election agents had been prevented from attending the count at several polling stations where the results then showed Zanu-PF doing significantly better than in surrounding areas.

But there was a growing acceptance among foreign monitors and diplomats that Tsvangirai had secured a clear victory over Mugabe.

South African monitors said they believed the opposition had won but would hold off on a public statement until the official results were announced. The Pan-African parliament observer mission warned against further delays in issuing the results.

A British foreign office minister, Mark Malloch-Brown, said it was "quite likely" that Mugabe had lost despite "massive pre-election day cheating".

Biti declined to say how the MDC will confront Mugabe if he refuses to give up power. But he repeated hints that while the MDC would stay within the law it would encourage its supporters to challenge the government on the streets.

"I'm not going to prescribe a formula for Zimbabweans. I'm going to speak for the party I lead. We're going to do everything legally and constitutionally," said Biti.