Tsvangirai says ZANU-PF is trying to sideline the opposition Movement for Democratic Change (MDC), adding no deal is possible unless party activists are released from jail.
The summit, which takes place in Pretoria, is being hosted by Kgalema Motlanthe, the South African president.
Al Jazeera's Haru Mutara, reporting from Harare, said: "The Zimbabweans are wondering what is it going to take, how long will they have to suffer, before regional leaders do something.
"This meeting could be the answer [or] it could not, but from the people I have spoken to, many people have given up hope."
A power-sharing deal, which allowed for Mugabe to remain president and for Tsvangirai to take the prime minister's post, was originally signed by the rival parties more than four months ago.
But it has not yet come into force because of the failure to agree on crucial ministerial positions.
With Mugabe engaged in tough bargaining with the opposition, the European Union has imposed new sanctions on Zimbabwe, where a cholera epidemic has led to the deaths of almost 2,800 people.
Western governments have repeatedly called for Mugabe to step down and are also pushing for major economic reforms before aid is offered.
Without a political settlement, it is also unlikely that sanctions imposed on Zimbabwe will be lifted.
The failure to find an agreement has worsened the humanitarian and economic crisis in the country, where more than half of the population is dependent on food aid.