Tsvangirai claimed on Saturday that he was the outright victor of the presidential battle, saying he had passed the 50 per cent threshold needed to avoid a second vote.
The MDC has accused Mugabe of delaying the results to orchestrate a run-off election.
Levy Mwanawasa, the Zambian president, has said that resolving the crisis would require a concerted effort by the region's leaders.
The opposition was awarded control of parliament in the simultaneous legislative elections, although ZANU-PF has called for a recount in enough seats to reverse that result.
Heads of state of the 14-nation Southern African Development Community (SADC) have been called to Zambia in a bid to break the impasse between Mugabe's ruling party and the opposition.
Biti has repeated a call from the MDC for the regional body to demand that Mugabe quit.
"We hope that the SADC meeting on Saturday will tell Mugabe to step down and to concede defeat," he said.
Biti confirmed on Thursday that Tsvangirai had been invited and would attend.
Mugabe to attend
Although the Zimbabwean government stopped short of confirming Mugabe's presence in Lusaka, it said he would attend if invited.
"If there is a SADC meeting confirmed by Zambia, President Mugabe will definitely be there," Bright Matonga, Mugabe's deputy information minister, said.
"There is nothing unusual about his attendance. SADC has obviously come under a lot of international pressure over the Zimbabwe elections and needs to be briefed about what is happening here."
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.