"We don't know why the world has to wait until dead bodies start littering the streets of Harare"
Tendai Biti, MDC secretary general
The move to hold a summit came as a Harare judge said he would deliver his judgement on Monday on an opposition petition demanding the release of election results.
A lawyer for Zimbabwe's electoral commission earlier said that it would be "dangerous" for the high court to order the release of presidential election results as demanded by the opposition Movement for Democratic Change.
The MDC has said its candidate, Morgan Tsvangirai, won the March 29 vote outright and accused Mugabe of delaying the results to orchestrate a run-off election.
'De facto military coup'
Tsvangirai said on Wednesday that troops had been deployed across Zimbabwe to intimidate people ahead of the run-off.
"The military leaders in the establishment are trying to subvert the will of the people," he told Time magazine.
"This is, in a sense, a de facto military coup."
Tsvangirai has embarked on a trip around the region, beginning with Botswana, to encourage leaders to help end the standoff.
Tendai Biti, MDC secretary general, said that regional leaders should push for Mugabe's resignation at the summit in Zambia.
"We don't know why the world has to wait until dead bodies start littering the streets of Harare," he said.
But Patrick Chinamasa, the justice minister who lost his seat in the recent parliamentary elections, brushed off calls for international intervention.
"Nothing has happened in Zimbabwe to warrant intervention by the UN. There is no justification to internationalise Zimbabwe," he said.