Movement for Democratic Change general secretary Tendai Biti said Wednesday that the result means there is no need for an election runoff against President Robert Mugabe.
The announcement came as the state newspaper said there would likely be a runoff vote - a candidate needs 50 percent plus one vote to avoid a runoff.
The latest official election results, though, show Zimbabwe's ruling party has lost its control of parliament. The results show the MDC has won 105 seats in the House of Assembly so far, while the ruling party has won 93 seats.
The official results had not been released, and the MDC threatened earlier in the day that it would publish its own figures if the elections commission failed to come forward with the official tally.
The MDC's victory claim was the most direct application of pressure on Mugabe to leave since the weekend election, and the state newspaper's prediction of a runoff was the first official admission that the country's autocratic leader of 28 years has failed to win re-election outright.
Independent monitors and Western governments said election results show the opposition Movement for Democratic Change, led by labor leader Morgan Tsvangirai, won a comfortable majority over President Robert Mugabe's Zanu-PF party.
Zimbabwe's state newspaper, The Herald, suggested the election was close.
"The Zimbabwe Electoral Commission yesterday released more election results with indications that Zanu-PF and the MDC Tsvangirai faction are headed for a tie in the House of Assembly poll, while the pattern of results in the presidential election shows that none of the candidates will garner more than 50 percent of the vote, forcing a re-run," the newspaper reported, prompting the MDC announcement.
"For years we have trod a journey of hunger, pain, torture and brutality," he said. "Today we face a new challenge of governing and rehabilitating our beloved country, the challenge of giving birth to a new Zimbabwe founded on restoration not retribution, on love not war."
In campaigning, the 84-year-old Mugabe had likened the elections to a boxing match, with his party winning in a knockout. Mugabe has been silent since the vote.
Washington has indicated it believed the opposition had won. "It's clear the people of Zimbabwe have voted for change," National Security Council spokesman Gordon Johndroe said.
The European Union said it wants Mugabe to step down.
The Electoral Commission has offered no results in the presidential race, saying Tuesday it still was receiving ballot boxes from the provinces. That raised questions about where those boxes have been since Saturday night, when some electoral officials slept on the ground to guard the votes.
Speculation was rife that Mugabe loyalists were trying to buy time to rig results, even as people close to the electoral commission and the opposition reported secret negotiations to allow Mugabe to exit gracefully.
Tsvangirai, 56, denied Tuesday he was in talks with Mugabe, saying he was first waiting for an official announcement of results. He claimed to have won more than the 50 percent needed for a first-round victory.
But a businessman close to the electoral commission and a lawyer close to the opposition said the two men's aides were negotiating a way for Mugabe to cede power. Both spoke on condition of anonymity because of the sensitivity of the issue. Several diplomats said they had heard similar reports, but could not confirm secret talks were under way.
"There are no discussions," Tsvangirai said.
Tensions rose as the days passed without an official announcement on the presidential vote, and some people stayed home from work. A senior police officer, Wayne Bvudzijena, said on state radio: "Our forces are more than ready to deal with perpetrators of violence."
Paramilitary police stepped up patrols in the capital Harare and Bulawayo, the second-largest city, and checked vehicles at roadblocks leading to the capital. Police ordered liquor stores and beer halls to shut early Tuesday night. The opposition has most of its support in urban centers.
Archbishop Desmond Tutu of South Africa, a Nobel peace laureate, said he feared violence.
"I would be very fearful of demonstrations ... given the brutality with which the authorities have in the past reacted," Tutu said Wednesday. "Many, many people are angry. I doubt that they are just going to sit back and fold their arms. They are going to take to the streets."
Political analyst John Makumbe said he learned from military officials that they would respect the vote results. The day before the elections, security chiefs said they would not tolerate an opposition victory.
The Zimbabwe Election Support Network, a coalition of 38 civil society organizations, said its random representative sample of polling stations showed Tsvangirai won just over 49 percent of the vote and Mugabe 42 percent. Simba Makoni, a former Mugabe loyalist whose rebellion brought divisions among the elite into the open, trailed at about 8 percent.
At independence, Mugabe was hailed for his policies of racial reconciliation and development that brought education and health to millions who had been denied those services under colonial rule. Zimbabwe's economy thrived on exports of food, minerals and tobacco.
The unraveling began when Mugabe ordered the often-violent seizures of white-owned commercial farms turned over to blacks, mainly relatives, friends and cronies who allowed cultivated fields to be taken over by weeds.
Today, a third of the population depends on imported food handouts. Another third has fled the country and 80 percent is jobless. Life expectancy has fallen from 60 to 35 years and shortages of food, medicine, water, electricity and fuel are chronic.