Zimbabwe's ruling regime stepped up its win at all costs election campaign with a firebomb attack that killed four opposition party activists.
The killings in Harare came even as South African president Thabo Mbeki was in the country for talks with president Robert Mugabe ahead of next week's run-off ballot.
The opposition Movement for Democratic Change party has said more than 60 of its activists have been killed in recent weeks, and accuses Mugabe of unleashing widespread violence to ensure victory over Morgan Tsvangirai.
Independent human rights activists have implicated police and soldiers as well as Mugabe party militants in the violence.
An MDC spokesman said that militants linked to Mugabe's party were seen in the area before the firebomb attack on the home of one party activist. He said the activist and three colleagues were killed, an unusually high one-day toll.
Mugabe "is behaving like a warlord," he said. "This violence must stop."
Mr Mbeki held talks with Mr Tsvangirai yesterday and then late into the night with Mugabe amid increasing international concern that the 27 June run-off will not be free and fair.
Mr Mbeki, who has continually refused to publicly rebuke Mugabe, left the country without commenting.
A Mugabe spokesman was quoted in today's state newspaper The Herald as saying Mr Mbeki came merely to review election preparations.
Mr Mbeki says confrontation with Mugabe could backfire. But the South African leader's decision to spend his 66th birthday with the 84-year-old Zimbabwean autocrat underlined the immense pressure he is under at home and abroad.
Mr Mbeki is being urged to take a tougher stance or to show that his quiet tactics can work to persuade Mugabe to stop the violence before the election; possibly even to bring Mr Tsvangirai and Mugabe together in a power-sharing compromise.
Mr Tsvangirai has called on Mr Mbeki to step down as mediator, accusing him of bias toward Mugabe.
Jacob Zuma, head of the African National Congress, has said the run-off election was unlikely to be free.
Most observers have praised the conduct of the first round - although not the delay in releasing official results. But there are growing fears that Mugabe will steal the second round through violence and ballot rigging.
In addition to the violence, Mr Tsvangirai's party has had rallies banned and campaign stops blocked by police. It has had little access to state media.
Mugabe's government earlier this month ordered humanitarian groups to suspend work in the country, accusing them of helping his opponents. That has put hungry Zimbabweans at the mercy of government feeding programmes.