"A runoff election could finally knock out the dictator for good," Tsvangirai said. "I am ready and the people are ready for the final round."
No runoff date has been set. Tsvangirai said Saturday it should be held within 21 days of the May 2 announcement of presidential results, but Zimbabwean government officials have said the electoral commission has up to a year to hold the vote.
Tsvangirai said he will return shortly to Zimbabwe and intends to "begin a victory tour." He maintains he won the first round outright and that official figures showing a second round was necessary were fraudulent.
Opposition officials and independent human rights activists have accused Mugabe of orchestrating violence against the opposition since the first round on March 29. Tsvangirai and other top opposition figures have stayed out of Zimbabwe since the initial voting.
"If this is going to be a successful runoff, opposition leaders and supporters must be able to freely campaign free of violence," said Gordon Johndroe, a spokesman for the U.S. National Security Council. "We would like to see election monitors - U.N. human rights monitors to ensure we have a safe electoral process there."
Tsvangirai left soon after the news conference for a meeting in Luanda with President Jose Eduardo dos Santos of Angola, who heads the Southern African Development Community election observer mission.
Observers inside and outside Zimbabwe have questioned whether a second round of voting could be free and fair with the opposition unable to campaign freely because of attacks and threats. Mugabe's ruling ZANU-PF, meanwhile, already has launched its runoff campaign.
Tapiwa Mudiwa, a 26-year-old supporter of Tsvangirai's Movement for Democratic Change, was skeptical Saturday.
"How are we going to campaign in the runoff as MDC supporters?" Mudiwa said in Harare, the Zimbabwean capital. "We can't wear MDC T-shirts. We fear we can't even go for rallies. Cars are being burned."
Tsvangirai acknowledged the risks and said another election "may bring more violence." But consultations with a wide range of Zimbabweans had convinced him they wanted him to run.
"They believe that we as a nation are brave enough, we are strong enough and we are angry enough to fight an election once again," he said. "We believe our people would feel betrayed if we shied away from the final knock out."
The Zimbabwe Association of Doctors for Human Rights has said 22 people have died and 900 tortured in postelection violence, while 40,000 farmworkers have been displaced in an effort to prevent them from voting in the run-off.
Tsvangirai said the violence, intended to "decimate" his party's election machinery, had had "some effect" but not disabled it. "We are going to ensure that we make the necessary preparations to overcome those obstacles," he said.
He called for SADC to ensure the runoff was held free of violence and monitored by regional peacekeepers, with unfettered access for international observers and journalists, many of whom were barred during the first round. He also said a new electoral commission should be established for the vote. These are "the optimum conditions" under which the runoff should be held.
"But we have stated that we are going to run," he said at the news conference, which also was attended by other top officials of his party.
Tsvangirai acknowledged some in Zimbabwe may have felt he had abandoned them. There have been persistent rumors he had gone into exile, though he has maintained he was traveling only to rally international support for democracy in Zimbabwe and always planned to return.
Fisher Murambatsvina, a 28-year-old MDC activist, said it was risky for Tsvangirai to return.
Tsvangirai, a former trade union leader, has survived three assassination attempts, including a 1997 attempt by unidentified assailants to throw him from a 10th floor window. Last year, he was hospitalized after a brutal assault by police at a prayer rally, and images seen around the world of his bruised and swollen face have come to symbolize the challenge dissenters face in his homeland.
"They beat him up before and this may happen again, just to break him down," Murambatsvina said Saturday in Harare. "It's risky for Morgan Tsvangirai to come back. The army is in charge. Right now, I don't think he will be safe if he is coming to start his campaign."
Mugabe, 84, has ruled Zimbabwe since independence in 1980 and once was hailed for promoting racial reconciliation and bringing education and health care to the black majority. But in recent years he has been accused of holding onto power through elections that independent observers say were marred by fraud, intimidation and rigging, and of overseeing his country's economic collapse.
"Mugabe was once my hero, too," Tsvangirai said Saturday. "It is very, very sad for me to call Mugabe a former liberator. It is sad for me to say that he has turned his back on both his people and his continent."