HARARE, Zimbabwe (CNN) -- Zimbabwe opposition candidate Morgan Tsvangirai said Sunday he would not participate in Friday's presidential runoff, provoking dismay from international observers and handing an apparent victory to President Robert Mugabe.
Tsvangirai said his Movement for Democratic Change party decided to pull out of the vote because of violence and arrests targeting his party and its supporters.
"A free and fair election is impossible," Tsvangirai told reporters Sunday. "We in the MDC have resolved that we will no longer participate in this violent, illegitimate ... sham of an election process."
Mugabe, who has been Zimbabwe's only leader since it gained independence from Britain in 1980, was facing his toughest re-election battle yet. Tsvangirai led Mugabe in the March 29 election, but failed to win enough votes to avoid a runoff. Learn more about Zimbabwe »
Patrick Chinamasa, Zimbabwe's justice minister, denied the MDC's allegations of intimidation and said Tsvangirai was dropping out to avoid "a humiliating defeat." Watch a businessman explain the situation in Zimbabwe »
But Britain, the former colonial power in Zimbabwe, blasted Mugabe's government for claiming a "tainted" victory built on "state-sponsored violence."
"I think the people around Robert Mugabe realize that power is ebbing away, and it's vital that they realize that there is no future other than a bleak one for a Mugabe regime that seeks to hang onto power at the barrel of a gun," British Foreign Secretary David Miliband said. Watch more about the impact of Tsvangirai's decision »
The Southern African Development Community has called on Mugabe to postpone Friday's vote. And in a statement issued Sunday evening in New York, U.N. Secretary-General Ban Ki-moon's office called for an immediate end to "the campaign of violence and intimidation that has marred this election."
"The secretary-general deeply regrets that, despite the repeated appeals of the international community, the government of Zimbabwe has failed to put in place the conditions necessary for free and fair runoff elections," Ban's office said.
"The circumstances that led to the withdrawal of opposition leader Morgan Tsvangirai today from the presidential elections represents a deeply distressing development that does not bode well for the future of democracy in Zimbabwe."
Tsvangirai announced the decision to withdraw following a closed-door MDC meeting Sunday. The party was apparently split on the decision, with an MDC official saying Friday that boycotting the runoff "would be giving into a violent dictator who is prepared to wage war on his own people to stay in power."
But Tsvangirai said a spate of arrests targeting MDC members, allegations of vote-rigging and attacks on the party's followers -- including an attack on an MDC youth rally by followers of Mugabe's ruling ZANU-PF party in Harare's stadium -- convinced him that a free and fair vote was impossible.
Chinamasa said the decision means Mugabe will win re-election by default, with no need for the vote scheduled for Friday.
"The candidate who remains in the race is formally declared the president," he told CNN, citing Zimbabwean law.
Chinamasa said Tsvangirai must formally submit his withdrawal from the race to Zimbabwe's Electoral Commission. He denied the MDC leader's charges.
"What is true is that ... Morgan Tsvangirai realizes that he's going to face a humiliating defeat on the 27th (of) June and he's trying to avoid that humiliating defeat," Chinamasa said.
Since the March 29 elections, there has been an uptick of attacks that Tsvangirai said has killed about 70 MDC supporters, displaced 200,000 Zimbabweans inside the country and destroyed 25,000 homes. There have been numerous reports from the opposition and church groups about kidnappings, torture and other violence, including the deaths of opposition party members.
Tsvangirai was among the MDC leaders picked up by police ahead of the runoff, and MDC Secretary-General Tendai Biti has been jailed on a charge of treason -- which can carry the death penalty. African and international leaders have criticized Biti's arrest, characterizing it as a ploy by Mugabe supporters to intimidate the opposition party before the vote.
Tsvangirai announced his decision after youth militias loyal to Mugabe's ZANU-PF ruling party attacked people at an MDC campaign rally in Harare's stadium, according to an opposition official and a journalist on the scene. Hundreds of rowdy youths dressed in ZANU-PF regalia occupied the venue, and the MDC said two of its reporters were seriously injured in the fights that ensued. Watch youth militias chase women, children »
"This country belongs to ZANU-PF," one youth shouted as he punched the air with a fist. "It came through blood and we are prepared to spill more in order to defend it."
Police had banned the rally last week, citing security concerns, but Zimbabwe's High Court on Saturday cleared the way for it to take place with a judge's order that banned national police from interfering.
Mugabe and other government officials have rejected accusations that government soldiers have targeted the opposition, and blame the MDC for inciting the pre-election violence. But video shot by U.S. Embassy officials last week showed women and children being chased by armed Mugabe supporters.
Mugabe has vowed not to leave office until all white-owned land is returned to the country's black majority, the state-run Herald newspaper reported Friday. Under land redistribution policies he started in 2000, Mugabe seized white-owned farms and gave the land to black Zimbabweans, saying they were cheated under colonial rule.
The number of white-owned farms in Zimbabwe, once in the thousands, has dwindled to about 400. Most of the redistributed land has not been harvested, and many analysts blame Zimbabwe's economic collapse -- including staggering inflation and unemployment -- on the farm seizures.
Mugabe recently warned that veterans he commanded in his country's liberation war nearly three decades ago would take up arms again if Tsvangirai were to win. He has also promised to arrest more officials from the MDC.