The MDC, which won the March 29 parliamentary elections and claims its leader, Morgan Tsvangirai, also won the presidential vote outright, said that it had not been invited to the talks between Mbeki and Mugabe.
They have called for Mbeki to be dismissed as a mediator due to his softy-softly approach towards Mugabe.
'No confidence in Mugabe'
Tsvangirai has said that he "has no confidence in Mbeki" and voiced his approval of Levy Mwanawasa, the Zambian president, taking over mediation.
Mwanawasa has been more critical of Mugabe than Mbeki, who does not believe that the Zimbabwean president would respond well to confrontation.
The electoral commission said last week that Tsvangirai had won more votes than Mugabe in the presidential vote, but failed to gain the necessary majority for a first-round victory.
Therefore, a run-off poll will be used to decide the contest, for which Mugabe has already started campaigning.
The MDC is expected to make an announcement on Saturday in South Africa whether it will enter the second poll, which has not yet been assigned a date.
Mugabe has been accused of organising violent attacks on the opposition since the elections and many fear that a run-off vote will not be free or fair.
'Increases in violence'
Reports are continuing from Zimbabwe of increases in violence in the country.
The Zimbabwe Association of Doctors for Human Rights said on Friday that 22 people had died and 900 were tortured in post-election violence.
But "violence is now on such a scale that it is impossible to properly document all cases," the association said in a statement, citing a "dramatic increase" in violence since the start of May.
"The level of brutality and callousness exhibited by the perpetrators is unprecedented."
The US ambassador in Harare visited a private clinic treating victims of political violence on Friday.
The victims were aged between four and over 80-years old according to a US embassy statement.
The MDC has said that 30 of its supporters have been killed since the March elections, with thousands more being wounded or tortured.
The government has refuted the claims, saying that opposition groups are behind attacks.