Meanwhile, a Zimbabwean magistrate has refused to throw out a treason charge against a senior MDC member.
Mishrod Guvamombe ruled on Friday that Tendai Biti, secretary-general of the MDC, would remain in custody until July 7.
"Taking into account all submissions, I'm of the view that there is reasonable suspicion to believe the accused committed the said offenses. Accordingly the application is dismissed," Guvamombe said in a Harare court.
Biti was first taken to court on Thursday to hear the charges against him, which include subversion and vote rigging.
The charges could carry the death penalty.
|MDC supporters are assessing whether or not Tsvangirai should contest the run-off [AFP]
Tsvangirai defeated Mugabe in a March 29 presidential election but, according to official results, he failed to win an outright majority, prompting the run-off vote.
The opposition, human rights groups and Western nations have accused Mugabe's Zanu-PF of orchestrating a violent campaign of intimidation to extend his 28-year rule.
The MDC says that at least 70 opposition activists have been killed and thousands more beaten by Zanu-PF groups and security forces.
Mugabe's officials blame the opposition for the bloodshed.
EU sanction threat
Leaders from the European Union are set to threaten Zimbabwean authorities with more sanctions amid reports of escalating violence against the opposition.
"The European Council reiterates its readiness to take additional measures against those responsible for violence," a draft text to be adopted at the end of a two-day summit in Brussels said.
The document, drawn up by EU diplomats overnight, did not specify what the additional measures would be.
A free and fair election is critical to the resolution of a political and economic crisis in the former British colony, the document said.
EU leaders urged the Southern Africa Development Community and the African Union to deploy a significant number of election monitors.
They also called for a swift and transparent vote count after lengthy delays in the first round.
EU sanctions currently include an arms embargo, visa bans and freezing of assets on more than a hundred officials, including Mugabe.