German Chancellor Angela Merkel added to growing pressure on Mugabe, who faces the toughest challenge to a rule that critics say has relied on tough security crackdowns and an elaborate patronage system.
"I think the situation for the people (in Zimbabwe) is unacceptable. We want a fair election result," she said at a news conference with Rwandan President Paul Kagame.
Zimbabweans just want relief from shortages of basic goods and the world's highest inflation rate of 165,000 percent.
Zuma and Brown promised that would come in the form of humanitarian aid and international efforts to secure Zimbabwe's economic recovery once the election process ends.
AFRICAN LEADERS
Zuma, who has distanced himself from the "quiet diplomacy" of South African President Thabo Mbeki over Zimbabwe, has called on African leaders to take action to unlock the stalemate.
Zimbabwe's neighbours, previously passive despite the collapse of the country's economy, took a harder line towards Mugabe this week, refusing to allow a Chinese ship to unload arms headed for the landlocked country.
Pro-government commentator Obediah Mukura Mazombwe add to uncertainty by suggesting Mugabe should lead a transitional government to end the deadlock while new polls were organised.
He said the solution should be mediated by Zimbabwe's neighbours. But analysts said Mugabe and his ZANU-PF party were pressing ahead with plans for a runoff vote against Tsvangirai.
Mazombwe holds no position in the ruling ZANU-PF party and his comments may not have official backing, analysts say.
In another opinion piece on Thursday, Herald political reporter Mabasa Sasa said a unity government was not feasible because ZANU-PF's radical nationalist policies were sharply different from the MDC's pro-Western stance.
Tsvangirai pressed ahead on a relentless regional drive seeking help from leaders to push aside Mugabe. On a visit to Mozambique on Wednesday, he rejected the idea of national unity government but said there were other options.
"The government of national unity does not arise because we won outright," he told a news conference.
The MDC, human rights groups and Western powers accuse ZANU-PF of launching a campaign of post-election violence. Tsvangirai says 10-15 MDC supporters have already been killed.
