Mr Mugabe, 84, has ruled Zimbabwe since independence in 1980
Talks in Zimbabwe aimed at breaking the deadlock over the formation of a unity government have collapsed.
Southern African mediators said after 12 hours there had been no agreement between President Robert Mugabe and opposition leader Morgan Tsvangirai.
Mr Tsvangirai called the impasse "the darkest day" but Mr Mugabe said he hoped for a new round of talks.
Both sides said earlier the talks would be the last chance to save a power-sharing deal signed four months ago.
The Southern African Development Community (SADC) led the efforts to save the deal and the group is to hold a crisis summit in Harare next week.
Mr Mugabe accused Mr Tsvangirai of presenting proposals which differed from recommendations by the 15-nation SADC.
Under September's deal, Mr Tsvangirai would serve as prime minister while Mr Mugabe remained president.
The deal first faltered after Mr Tsvangirai's Movement for Democratic Change (MDC) accused Mr Mugabe's Zanu-PF of trying to keep the most powerful ministries.
The political deadlock has exacerbated the problems facing Zimbabweans, from a cholera epidemic and an economic meltdown to food shortages and the collapse of basic services such as health and education.
"For us as the MDC this is probably the darkest day of our lives," Mr Tsvangirai said.
STATE OF ZIMBABWE
Five million people - almost half population - need food aid
Central bank introduced Z$100tr note, worth about US$30 (£20)
Unemployment more than 80%
More than 2,200 people have died in cholera outbreak
"I am sure the whole nation is waiting anxiously for the resolution of this crisis. We are committed to this deal but subject to Zanu-PF conceding on these issues."
Mr Mugabe said he would seek further talks before the SADC summit.
"We shall continue to exchange ideas and see where the differences are with the SADC proposal," he added.
Zimbabwean journalist Brian Hungwe - who was at the talks taking place in a luxury Harare hotel - said Mr Tsvangirai had presented a thick dossier of new conditions at the meeting, prompting an angry response from Mr Mugabe.
South African President Kgalema Motlanthe, his predecessor Thabo Mbeki and Mozambican leader Armando Emilio Guebuza attended the talks, pushing for a breakthrough.
Also attending was Arthur Mutambara, the head of an MDC breakaway faction who is supposed to become deputy prime minister under the pact.
On the eve of the meeting, Mr Mugabe said he had done everything required under the agreement and the time for talks was over.
Mr Tsvangirai (C) had been out of Zimbabwe since November
The clear implication was that if the MDC did not signal agreement, Mr Mugabe would go ahead and form a government unilaterally, the BBC's Peter Biles in Johannesburg says.
The MDC wants a share of the security ministries, ambassadors and regional governors.
Under the deal, it has 13 ministries to 15 for Mr Mugabe's Zanu-PF but under the existing proposals, Zanu-PF would hold national security and defence, while home affairs, which runs the police, would be shared.
The MDC has also demanded an end to the abduction of opposition and human rights activists by state security agents.
Mr Tsvangirai arrived back in Zimbabwe on Saturday after an absence of more than two months, telling reporters his party would not be "bulldozed" into any deal not reflecting the will of the people.
Mr Tsvangirai gained the most votes in elections last March but not enough for outright victory.
He pulled out of a run-off in June against Mr Mugabe, citing a campaign of violence against opposition supporters.