At a news conference, Mr. Tsvangirai, who leads the opposition Movement for Democratic Change, or M.D.C., said he was unwilling to ask the party's supporters to go to the polls on Friday "when that vote will cost them their lives."

Mr. Tsvangirai's decision came on a day when governing party youth militia armed with iron bars, sticks and other weapons beat his supporters as they sought to attend a rally for him in Harare.

It was the latest incident in a tumultuous campaign season in which Mr. Tsvangirai has been repeatedly detained, his party's chief strategist has been jailed on treason charges most observers believe are trumped up, and rampant state-sponsored violence has left at least 86 dead and thousands injured, according to tallies by doctors treating the victims.

Over the past week, a growing chorus of leaders in Africa and abroad have declared unequivocally that a free and fair election is now impossible in Zimbabwe.

There are growing cracks in the longtime solidarity of African leaders with Mr. Mugabe, a liberation hero whose defiant anti-western rhetoric for years struck a chord in the region, but it remains to be seen whether Zimbabwe's neighbors will censure Mr. Mugabe or take even tougher steps, such as economic sanctions, to isolate his regime.