The offer was made by two of the ruling party's most powerful figures -- Parliament Speaker Emmerson Mnangagwa and armed forces chief of staff General Vitalis Zvinavashe -- and was an effort to help Zimbabwe regain international legitimacy and renewed aid and investment during a period of transitional rule, the mediators said.
The mediators, fearing allegations of treason if the deal collapses, said assurances Mugabe would step down were conveyed to the opposition Movement for Democratic Change.
A power-sharing government would try to end an economic meltdown that has sent inflation soaring, caused a massive fuel shortage and left at least half Zimbabwe's population on the verge of starvation.
Mugabe, who led the nation to independence in 1980, won a new six-year term in elections last March that independent observers said were deeply flawed. The early retirement of Mugabe, once seen as a towering African statesman, has long seemed inconceivable.
MDC leader Morgan Tsvangirai confirmed receiving the offer and, in a departure from recent opposition policy, said his party's lawmakers were ready to vote with the ruling party for a constitutional amendment allowing the creation of a caretaker government once Mugabe stepped down.
Any agreement would include guarantees of immunity for Mugabe, 78, from prosecution over alleged misrule and human rights violations during his 23 years in power, Tsvangirai said.
Ruling party officials were unavailable for comment Sunday.
There has been no word on an offer from Mugabe himself, who was scheduled to head home from a two-week vacation that included a trip to Thailand. He was expected to return to his office Monday.
His absence as the nation faced food and gasoline shortages has fanned harsh criticism at home.
"There is wide consensus [that] Mugabe is the problem and national and party dialogue must begin. Colleagues have shifted the blame onto him and he must accept the consequences," said one mediator who spoke on condition of anonymity.
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