The opposition announcement came after thousands of police in riot gear and soldiers blockaded the site of the MDC's main campaign rally.
Had Tsvangirai originally chosen to boycott the June 27 run-off, he would have set the stage for Robert Gabriel Mugabe, the president, to win re-election by default.
Tsvangirai's supporters would also have viewed him as a coward and a let-down, Zimbabwean political analysts say.
But contesting the run-off, perhaps the second-best option, represented major hurdles - the MDC were always aware of the likelihood of intimidation and violence against people who voted for them in the first round.
Tsvangirai has been detained by police several times, and released without charge, during his election campaigning.
Tendai Biti, the MDC's secretary-general, has also been arrested and is facing charges of treason, which carry a death sentence upon conviction.
Tsvangirai's supporters have not only been attacked and beaten but they have also been murdered for opposing Mugabe, according to the MDC, who have put the death toll at at least 80.
Mugabe has sworn that his supporters
will fight to keep him in power [AFP]
Local election monitors say some MDC supporters have been forcibly removed from their homes, further hindering their abilities to vote; scores more have been hospitalised with severe injuries.
Tsvangirai himself has been to see supporters in hospital who allegedly have been assaulted by security forces and supporters of Zanu-PF, the governing party headed by Mugabe.
For several weeks, the Zimbabwe Election Support Network (ZESN), which deployed hundreds of observers across the country in the March 29 election, has released statements highlighting incidents of violence against MDC supporters and some of its observers.
But there were hopes that Tsvangirai would win the run-off in spite of a vow by Mugabe that the opposition would never come to power as long as he is alive.
Mugabe has also said that the country's war veterans are ready to fight to try to prevent the MDC from coming to power.
Political observers and analysts say Zimbabwe's deteriorating economic situation made the MDC a popular alternative to Mugabe.
|Economic travails could play into the
hands of the MDC opposition [AFP]
"Mugabe could go on and try every trick in the book, but the sheer number of voters [for the MDC] may be hard to beat," Wilf Mbanga, the London-based editor of The Zimbabwean, a weekly printed in South Africa and sold in Zimbabwe, says.
Others say intimidation may work to the disadvantage of Mugabe.
"Intimidation could turn out to be counterproductive," Professor Tony Hawkins, who lectures at the University of Zimbabwe, told Al Jazeera.
"But chasing people from their homes will make it impossible for them to vote. If you’re chased away, you can't vote."
Mugabe's controversial land redistribution program - which began in 2000 and saw white farmers forced off their farms, some of which went to Mugabe cronies who have no knowledge on how to run them - has hampered food production and brought Zimbabwe to its knees.
In an attempt to stave off the crisis, the government has been printing money with planeloads of banknotes arriving in Harare, the capital, almost on a weekly basis, according to the UK's Sunday Times newspaper.
This has resulted in runaway inflation, which stood at 165,000 per cent in February; Zimbabwe has possibly the largest number of millionaires in Africa, but they can't even afford basic commodities.
The Zimbabwean dollar is almost worthless. A one million-dollar bill can only buy a few items in supermarkets - if the customer carrying it is lucky to find goods on the shelves.
Unemployment among the country's 13 million people is said to be around 80 per cent, and hundreds of thousands of Zimbabweans have had to seek economic refuge in neighbouring South Africa, Botswana and Mozambique.
They can return home if Tsvangirai one day takes power and sorts out the economic mess, but the former trade union leader has several hurdles yet.