Robert Mugabe's party is demanding a recount of some votes
President Robert Mugabe and opposition leader Morgan Tsvangirai are to attend an emergency summit of regional leaders to discuss Zimbabwe's recent election.
Saturday's talks in Zambia were called amid the failure of Zimbabwe's election commission to publish results of the presidential election held 12 days ago.
The opposition has refused to take part in any second round run-off vote.
Mr Tsvangirai is in Botswana, where a minister quoted him as saying he had left a tense Zimbabwe to ask for help.
At a news conference in Johannesburg, South Africa, on Thursday, the Movement for Democratic Change said the delay of results amounted to "a constitutional coup d'etat".
MDC Secretary General Tendai Biti said: "We won the presidential election hands down, without the need for a run-off, so we will not participate in a run-off."
The MDC originally reported that Mr Tsvangirai had won 50.3% of the vote. Asked whether this was a big enough margin to declare victory, Mr Biti said the party had since obtained broader results suggesting it had done even better.
Mr Biti accused ruling Zanu-PF-backed militias of unleashing a campaign of violence across the country to intimidate rural voters ahead of a possible run-off.
It coincided with a similar charge made by Amnesty International, which accused Zimbabwean police and soldiers of "organised post-election violence aimed at opposition supporters".
Mr Biti said Mr Mugabe was a "caretaker president", adding: "So we are therefore concerned that an illegitimate government is now in place in Zimbabwe."
Zambian President Levy Mwanawasa, who chairs the 14-nation Sadc grouping, has called the weekend's emergency meeting.
ELECTION RESULTS SO FAR
None so far
Winner needs more than 50% to avoid run-off
MDC breakaway: 6
Zimbabwe's Information Minister Sikhanyiso Ndlovu said they were happy to brief Sadc, but added: "There is no crisis in Zimbabwe that warrants a special meeting on Zimbabwe."
Mr Tsvangirai wants other southern African leaders to put pressure on the Zimbabwean government to announce the results of the 29 March presidential election.
US President George W Bush on Thursday said Zimbabwe's authorities had to release the results "as soon as possible", the White House said.
A spokesman for Mr Bush said he made the call in a telephone conversation with Tanzanian President Jakaya Kikwete.
Independent and ruling party projections say Mr Tsvangirai did not win the 50% vote share needed to win outright.
South Africa dismissed any suggestion it would ask Mr Mugabe to step down.
Its deputy foreign minister said in Pretoria: "We are not a government who can ask other presidents to step down."
The MDC says there is no need for a presidential run-off
Meanwhile, Zimbabwe's state-run Herald newspaper reports the ruling Zanu-PF has increased the number of constituency results it is contesting from 16 to 21.
In the House of Assembly, Zanu-PF has lost its majority for the first time since independence, with 97 seats against the MDC's 99 in the 210-seat chamber. A smaller MDC faction has 10 seats.
The MDC is still hoping legal action in the High Court will lead to the immediate release of the presidential results. A ruling is due on Monday.