Zimbabwe's opposition party claimed an overwhelming victory against President Robert Mugabe in yesterday's presidential election, saying that the flow of results showed its candidate, Morgan Tsvangirai, had 'massacred' the ruling Zanu-PF party.

The Movement for Democratic Change (MDC) defied a government ban on pre-empting the official announcement of the election results and released the count from polling stations that showed Tsvangirai beating the man who has ruled Zimbabwe for 28 years, even in the president's home territory of Mashonaland.

'We've won this election,' said Tendai Biti, the MDC's secretary-general. 'The results coming in show that in our traditional strongholds we are massacring them. In Mugabe's traditional strongholds they are doing very badly. There is no way Mugabe can claim victory unless it is through fraud. He has lost this election.'

The government's electoral commission has yet to release the counts formally. But the MDC said that declarations posted at polling stations across Zimbabwe last night, and gathered from its agents observing the counts, showed Tsvangirai ahead of Mugabe in every province where results were available. The most dramatic gap was in Mashonaland West, where the MDC candidate had 88 per cent of the vote to the president's 12 per cent.

Even in rural areas, where Mugabe has traditionally commanded support, he was taking only half as many votes as Tsvangirai, according to the MDC. In Harare, the opposition candidate was pulling in three times as many votes as the president. It was not clear what proportion of the overall vote the results represented, but Biti claimed it was substantial and the trend was 'irreversible'.

He said the MDC was releasing the results ahead of the electoral commission to head off any attempt by the government to tamper with the figures when they are centrally collated, as they believe happened in the presidential election that Mugabe won by a narrow margin six years ago.

'We don't trust the electoral commission. It isn't independent. We made the mistake in 2002 of not claiming our victory,' he said. 'If they arrive at figures which are different, we will not accept that, pure and simple.'

Asked how the MDC will challenge any attempt to fix the figures, Biti hinted at popular protest. 'We will not make the same mistake again of taking court action. But we will remain restrained as a movement. The MDC will act inside the law. But the MDC cannot speak for the people of Zimbabwe,' he said.

The MDC's move came despite a warning by Zimbabwe's police chief, Augustine Chihuri, who said he will not permit the opposition to declare victory. He also warned against a plan by the MDC to call large numbers of people on to the streets to defend the result. Police were posted at all major intersections in Harare yesterday, while riot police were held in reserve.

The MDC's claim of victory came despite its protests about election irregularities earlier in the day. Poll monitors raised concerns at the large numbers of people who were told they could not vote because of errors on the electoral roll. African election monitors wrote to the electoral commission questioning the registration of more than 8,000 voters on vacant lots in Harare or crammed by their hundreds into small shacks. Before the vote, Tsvangirai said the MDC had uncovered evidence of widespread vote-rigging including the names of a million 'ghost' voters on the electoral roll.

Thousands of Zimbabweans slept at polling stations and queued for hours before they opened after the opposition called for a large turnout to counter efforts by Mugabe to rig the election.

The state-run Herald newspaper published a poll giving the president 57 per cent of the vote, enough to avoid a run-off. The opposition says that number is no more than a guide to the scale of the fraud the government has planned.