"The MDC ... will contest the run-off. I am ready, and the people are ready for the final round."
'Unfettered access' call
Tsvangirai said that full access for international observers and media was a pre-condition for his participation in the run-off against president Robert Mugabe.
Tsvangirai said: "We want unfettered access of all international observers. The ZEC [Zimbabwe Electoral Commission] has totally discredited itself to a point now they are partisan to the [ruling] ZANU-PF. The reconstitution of ZEC is therefore paramount.
"Media access should be unfettered, both print and international media," he said, listing the conditions for his participation in the run-off after disputed March 29 polls."
He also called for an end to violence and asked the Southern African Development Community to send peacekeepers to monitor the upcoming presidential run-off.
The opposition leader has said previously he won the first round outright and that official figures showing a second round is necessary are fraudulent.
The MDC maintained its leader won over 50 per cent of the first round vote, making a run-off unnecessary.
But official results gave him a smaller margin or victory over Mugabe, therefore, a run-off poll will be used to decide the contest, for which Mugabe has already started campaigning.
Under the terms of Zimbabwe's electoral law, any boycott by Tsvangirai would effectively hand victory to Mugabe who has ruled the former British colony since independence in 1980.
The MDC announcement comes as Thabo Mbeki, the South African president, ended a one-day visit to Zimbabwe for talks with Mugabi over the political crisis and violence following disputed elections in March.
Mbeki met Mugabe on Friday in Harare, as the MDC demanded that Mbeki step down as mediator in the crisis.
Mbeki was on his third visit on behalf of the Southern African Development Community, and was met at the capital's international airport where he walked hand-in-hand with Mugabe and was decorated with flower garlands.
The talks lasted for nearly four hours, after which Mbeki returned to South Africa.
He did not talk to any other government officials or opposition leaders.
The MDC have called for Mbeki to be dismissed as a mediator due to his "softy-softly" approach towards Mugabe.
Tsvangirai has said that he "has no confidence in Mbeki" and voiced his approval of Levy Mwanawasa, the Zambian president, taking over mediation.
Mwanawasa has been more critical of Mugabe than Mbeki, who does not believe that the Zimbabwean president would respond well to confrontation.
Violence is reported to be increasing in Zimbabwe.
The Zimbabwe Association of Doctors for Human Rights said on Friday that 22 people had died and 900 had been tortured in post-election violence.
But "violence is now on such a scale that it is impossible to properly document all cases," the association said in a statement, citing a "dramatic increase" in violence since the start of May.
"The level of brutality and callousness exhibited by the perpetrators is unprecedented."
The US ambassador in Harare visited a private clinic treating victims of political violence on Friday.
The victims were aged between four and more than 80-years-old according to a US embassy statement.
The MDC has said that 30 of its supporters have been killed since the March elections, with thousands more being wounded or tortured.
The government has refuted the claims, saying that opposition groups are behind attacks.