An official said regional leaders would discuss the stalemate between President Robert Mugabe and his rivals at a summit in either Botswana or South Africa next week, which all Zimbabwean leaders have agreed to attend.
"For us as the MDC, this is probably the darkest day of our lives," Tsvangirai said after a 12-hour meeting in Harare, which was billed as a last-ditch effort to save the pact.
A unity government is seen as the best chance of preventing total collapse in once-prosperous Zimbabwe, where prices double every day and more than 2,000 people have died in a cholera epidemic.
But a September power-sharing agreement has stalled amid fighting over who should control key ministries and regional leaders have failed to secure a compromise, despite international calls for stronger action.
Mugabe said talks had broken down but would continue, and blamed Tsvangirai’s opposition Movement for Democratic Change (MDC) for blocking proposals put forward by the Southern African Development Community (SADC) at the meeting.
Tsvangirai reiterated that the MDC was committed to the deal but only if Mugabe cedes control of powerful ministries, such as home affairs, finance and information.
South African President Kgalema Motlanthe, Mozambique leader Armando Guebuza and Thabo Mbeki, regional mediator and South Africa’s former president, attended the Harare meeting.
SADC executive secretary Tomaz Salomao said next Monday’s emergency summit would discuss Tsvangirai’s demands.
"After then, a public statement will be made," Salomao said.
The MDC said earlier abductions of its members, which it blames on Mugabe’s ZANU-PF, proved "beyond a shadow of a doubt" its rivals had not respected the spirit of the September pact.
Zimbabwe’s economic and humanitarian crisis has deepened during months of political stalemate and an aid group warned on Monday that hundreds of thousands of people could be at risk from a surge in cases of malaria, which can be deadly.
The United Nations-backed Roll Back Malaria Partnership said 1.63 million Zimbaweans were at increased risk of contracting malaria after usual prevention measures were neglected by health workers busy trying to contain the cholera epidemic.
Mugabe said on Sunday the MDC should accept the deal or end talks over its implementation, state media reported.
"Since they signed the deal, the parties are further apart and there is a strong likelihood of the total collapse of the political agreement," political commentator and Mugabe critic John Makumbe told Reuters.
"It is very likely that ZANU-PF will go it alone and form a minority government. It is also very likely that without an agreement, the humanitarian crisis will take a sharp turn for the worse."
Tsvangirai won a presidential election last March but by too few votes for an outright victory. He pulled out of the subsequent run-off, citing violence against MDC supporters.
Zimbabwe’s parliament is due to resume sitting on Tuesday.