The release of the opposition supporters comes as the UN Security Council prepared to meet in a closed session in New York to discuss the political situation in Zimbabwe.
Zimbabwe's divided opposition formally reunited on Monday, declaring that it has a majority in parliament against Zanu-PF, the party of Robert Mugabe, the president.
Morgan Tsvangirai, the leader of the MDC, announced "that our two formations in parliament have agreed to work together", referring to the MDC and a breakaway faction.
At a news conference in South Africa on Monday, Arthur Mutambara, the leader of the second faction of the MDC, said: "There will be no division amongst ourselves vis-a-vis the resolution of the Zimbabwean crisis.
Referring to Tsvangirai, Mutambara said: "I'm here to show solidarity to the winner of the presidential election in Zimbabwe."
The two MDC factions together control 109 seats against Zanu-PF's 97 seats, according to results from the March 29 elections.
A partial recount of results by Zimbabwe's electoral commission confirmed the opposition has a parliamentary majority.
Utoile Silaigwana, a spokesman for Zimbabwe's electoral commission, said recounts for five remaining constituencies were being collated and would be available on Tuesday.
|Zimbabwe's political opposition says the UN |
should address post-election violence [AFP]
"Once this process has been completed, the process of verification of the presidential ballots will also commence tomorrow," he said.
Tsvangirai has also proclaimed himself the winner of the presidential election held on the same day as the parliamentary vote.
He has said that that Mugabe should concede the presidency.
Results from the presidential vote have still not been released by the authorities.
But an election official said on Monday that verification of results from the presidential polls will start on Tuesday.
At the news conference in South Africa on Monday, Tsvangirai called on the UN to "send a special envoy to investigate this violence and recommend a way to resolve the issue".
"What has transpired is a militarisation of our country. We have requested that the UN must deal with this matter," he said.
The MDC says at least 15 people have been killed and scores more have been arrested since the presidential poll.
In a counter claim, Zanu-PF said one man was killed and two others were injured when MDC supporters tried to attack an army training camp, The Herald, a state newspaper said on Tuesday.
Mugabe accuses his critics of plotting with Western nations to end his 28-year rule.
Zimbabweans face severe shortages of basic goods and an inflation rate of 165,000 per cent - the world's highest.