Zimbabwe's opposition leader Morgan Tsvangirai has said he will contest a presidential run-off, despite fears of widespread poll violence.
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.
The opposition leader went on to demand "unfettered access of all international observers" and journalists covering the vote.
He also called for the deployment of southern African peacekeepers to curtail any violence.
The BBC's Peter Greste in Johannesburg says the announcement was not a big surprise, as boycotting the poll would have meant handing victory to President Mugabe by default.
Our correspondent says Mr Tsvangirai has to get home soon - as long as he remains outside Zimbabwe, it will be hard to maintain the support he needs to win the run-off.
The MDC leader said he expected to return to Zimbabwe in the next two days.
The Zimbabwe Electoral Commission has not yet set a date for the run-off.
Mr Tsvangirai said it should take place by 23 May - within three weeks of the first-round results being declared.
According to these official declarations, he won 47.9% of the vote, against 43.2% for Mr Mugabe.
Robert Mugabe has ruled Zimbabwe for close to 30 years
Although the first round was largely peaceful, the results were not announced until 2 May.
The MDC says the delay gave the authorities time to rig the counting and carry out attacks on its supporters in anticipation of a run-off.
A trade union official on Thursday said that 40,000 farm-workers and their relatives had fled their homes because of violent attacks
The MDC says at least 25 of its supporters have been killed since the first round, and hundreds have been forced from their homes in rural areas.
But police and officials from Mr Mugabe's Zanu-PF party have accused the MDC of staging attacks, while accusing the MDC of exaggerating the scale of the violence.
Mr Mugabe has been in office since independence in 1980.