Tsvangirai in Zimbabwe Run-Off
Speaking in South Africa, he said people would feel "betrayed" if he did not run, and vowed to return shortly.
Mr Tsvangirai called for an end to violence, as well as full access by international monitors and media.
Official first-round results put him ahead of President Robert Mugabe, but not by enough votes to win outright.
Mr Tsvangirai had earlier insisted he had secured more than 50% of the vote, and that there was therefore no need for a second round.
On Saturday, he told reporters that his Movement for Democratic Change (MDC) had faced a "very difficult" decision but that after consulting supporters in Zimbabwe, it had decided to take part.
"I am ready, and the people are ready for the final round," he said.
Itwas the first time the opposition leader committed to participating inthe run-off, which he says is not necessary because he believes he wonthe election outright.
"We want unfettered access of allinternational observers," he said, listing the conditions for hisparticipation in the run-off. "Media access should be unfettered, bothprint and international media."
He also called for the deployment of southern African peacekeepers to curtail any violence.
Zimbabwe's "war veterans" militia plan to intimidate voters by posing as police officers during the presidential run-off, a policeman has told the BBC.
He said they would be based inside polling stations during the vote, whose date has not yet been fixed.
It has been confirmed that police have arrested two top trade union officials.