The former South African president arrived in Harare in an attempt to salvage the deal he recently brokered.
Zimbabwean President Robert Mugabe has sworn in two vice-presidents ahead of talks on forming a cabinet, a government official said, a move that could further endanger power-sharing negotiations.
It follows Mugabe's allocation of important ministries to his ZANU-PF party at the weekend, angering the opposition. The MDC said it doubted mediation by Mr Mbeki today would be able to get ZANU-PF to compromise.
A senior government official told Reuters "The two vice-presidents were sworn in this morning because their positions are not in dispute."
Opposition Movement for Democratic Change leader Morgan Tsvangirai said yesterday his party could walk away from a power-sharing deal he signed with Mugabe if Mbeki's latest mediation effort failed to end a deadlock on how to divide key ministries.
"The visit provides a platform and opportunity for ZANU-PF to reverse its unilateral action," MDC spokesman Nelson Chamisa said. "The ZANU-PF mindset is not consistent with power-sharing. It cannot be power-sharing when one party controls all key ministries."
Mbeki scored his biggest diplomatic coup last month when he nudged Zimbabwe's bitter political rivals to sign a power-sharing deal.
A government notice on Saturday showed Mugabe had allocated three key ministries to his ZANU-PF party, drawing fire from the opposition and threatening the fragile pact.
Mugabe handed his party the ministries of defence, home affairs - which is in charge of the police - and finance which will be strategic in reviving the collapsing economy.
But the official Herald newspaper has suggested that the finance post was the only outstanding issue.
Chamisa said there was no agreement on all powerful ministries, including, justice, foreign affairs, information and local government.
Mbeki's role was thrown into doubt after his ruling ANC forced him to resign.
Tsvangirai said yesterday he would continue negotiating to try to reach an agreement but added that the country's 10 posts of provincial governors should be shared between ZANU-PF, a splinter MDC group and his party.
While the parties have been at loggerheads since the signing of the 15 September pact on how to divide up 31 cabinet posts, this has angered Zimbabweans who had hoped the deal would bring an end to years of economic misery.
Under the deal, Mugabe - in power since Zimbabwe's independence from Britain in 1980 - retains the presidency and chairs the cabinet. Tsvangirai, as prime minister, will head a council of ministers supervising the cabinet.
ZANU-PF will have 15 seats in the cabinet, Tsvangirai's MDC 13 and a splinter MDC faction led by Arthur Mutambara three posts, giving the opposition a combined majority.