Zimbabwe's ruling party lost control of parliament today and the opposition said it had also defeated veteran leader Robert Mugabe in a presidential vote.
Official figures said the combined opposition had taken 105 seats in the 210 seat parliament with one going to an independent. Mugabe's ZANU-PF has so far taken 94.
The mainstream Movement of Democratic Change faction of Morgan Tsvangirai said he had won 50.3 per cent of the presidential vote and Mugabe 43.8 per cent according to its own tallies of results posted outside polling stations.
MDC Secretary General Tendai Biti said this absolute majority was enough for outright victory but Tsvangirai would accept a second round run-off against Mugabe "under protest".
Biti appealed to Mugabe, president for the last 28 years, to concede defeat and avoid "embarrassment".
Mugabe, 84, faced an unprecedented challenge in Saturday's elections because of the economic collapse of his once prosperous country.
His government immediately rejected the MDC victory claim as "mischievous".
Deputy Information Minister Bright Matonga told Sky television: "President Mugabe is going nowhere. We are not going to be pressurised into anything."
The government has warned that victory claims before an official result would be regarded as a coup d'etat.
Matonga said in a telephone interview with Sky: "No-one is panicking around President Mugabe. The army is very solidly behind our president, the police force as well."
He added: "We are not going to be rushed by anybody. They can make statements left right and centre, but they are merely wasting their time."
No official results of the presidential poll have been issued four days after the election and Mugabe has not been seen in public since voting, despite speculation he would make a television address on Tuesday night.
Biti's announcement indicated a change in the MDC position in accepting a second round run-off against Mugabe. Tsvangirai said on Tuesday he had won outright.
Mugabe's government appears to have been preparing the population for a run-off by revealing its own projections showing a second round would be required in the statutory three weeks after last Saturday's vote.
Both Tsvangirai and the government have dismissed widespread speculation that the MDC was negotiating with ZANU-PF for a managed exit for Mugabe, who has ruled uninterrupted since independence from Britain in 1980.
The state-owned Herald newspaper said on Wednesday projections for the presidential election showed Mugabe would fail to win an outright majority for the first time in nearly three decades.
The prospect of a run-off has raised fears both inside and outside Zimbabwe that the hiatus before a new vote would spark serious violence between security forces and militia loyal to Mugabe on one side and MDC supporters on the other.
The Herald also said the government had decided to immediately implement tax relief to cushion the effect of runaway inflation, officially over 100,000 per cent but estimated to be much higher - the world's highest rate.
The widening of workers' tax-free threshold tenfold to 300 million Zimbabwean dollars per month - $10,000 at the government's official rate but about $7.50 on the black market - is widely seen as an attempt to curry favour with voters and suggests ZANU-PF is preparing for a run-off.
A senior Western diplomat in Harare told Reuters the international community was discussing ideas to try to persuade Mugabe to step down, "but I don't think there is anything firm on the table."
The opposition and international observers said Mugabe rigged the last presidential election in 2002. But some analysts say the groundswell of discontent over the economy is too great for him to fix the result this time without risking major unrest.
Apart from surreal inflation of more than 100,000 per cent and a virtually worthless currency, Zimbabweans are suffering food and fuel shortages and an HIV/AIDS epidemic that has contributed to a steep drop in life expectancy.
The opposition, including former Finance Minister Simba Makoni, who stood as a third candidate, is expected to unite behind Tsvangirai if there is a run-off.