(CNN) -- Political opponents of Zimbabwe President Robert Mugabe denounced Friday's presidential election results as "illegal" and said they stand by their stance that their leader won the race and that no runoff is necessary.
Morgan Tsvangirai has argued that a run-off is unnecessary and that the government would rig the outcome.
The Zimbabwe Electoral Commission said challenger Morgan Tsvangirai won 47.9 percent of the vote, compared with 43.2 percent for Mugabe.
"The announcement of the results today was illegal. The MDC stands by its previous stance that the vote was stolen from the opposition by a regime that is clinging to power," said Tendai Biti, secretary-general of Tsvangirai's political party, Movement for Democratic Change.
Under Zimbabwe law, because neither candidate got 50 percent plus one vote, a runoff is needed.
But Biti said the law states that the candidate who has the most votes should be president. The MDC also contends that Tsvangirai got 50.3 percent of the vote and is the official winner.
Results almost identical to the official ones were reported to CNN on Thursday -- the commission's first day of the verification process -- by a senior official with Mugabe's ruling Zanu-PF party who did not want to be named.
A delay in announcing the results of the March 29 election has prompted accusations of vote-rigging and calls from around the world for Mugabe to step down after 28 years in power.
The Movement for Democratic Change has repeatedly said that Tsvangirai won the election, but Zanu-PF alleges that the opposition has been engaged in election tampering. Read about the candidates in the presidential election. »
The MDC has also maintained that it will not participate in a runoff. If it holds to that, Mugabe would retain the presidency, said the chief of the electoral commission, George Chiweshe.
Reports of violence against opposition supporters have emerged from Zimbabwe amid heightened tensions since the presidential election.
Zimbabwe's religious leaders called for international help.
"People are being abducted, tortured, humiliated by being asked to repeat slogans of the political party they are alleged not to support," according to a statement from a coalition of Christian churches in Zimbabwe released two weeks ago. "In some cases, people are murdered."
Government spokespeople have denied those reports or said they were exaggerated.
A U.S. State Depertment spokesman said that there were "serious credibility problems" with the official results and that the United States planned to consult with other countries on the situation.
"It's really impossible, as a practical matter right now, to think about how Zimbabwe could hold a runoff election in a situation when everyone admits [that] by any measure, the leading vote-getter is having his party and his supporters regularly harassed and subject to abuse by government officials," Tom Casey said.
CNN and other major news organizations are banned from reporting from Zimbabwe, where there are reports of beatings and intimidation by the government against citizens who support the opposition.
Humanitarian groups said that 105,000 people have been displaced because of the violence, and there were questions about who would come out to vote in the runoff.
Mugabe, 84, is the only ruler Zimbabwe has had since British rule of the former Rhodesia came to end in 1980, and he was a hero of the civil war against the white government.
He has been re-elected several times, often either running unopposed or in elections that prompted charges of fraud and state-sponsored terrorism against opponents, and has consolidated his rule over all aspects of Zimbabwean life.
Two weeks before the last presidential election in 2002, which the MDC alleges was stolen, the government charged Tsvangirai with treason. He was acquitted. The MDC accused Mugabe of trying to eliminate him as a challenger.
Zimbabwe faced international sanctions after the 2002 election, including travel restrictions on Zimbabwean officials.
It was once known as the breadbasket of southern Africa, but it is now difficult to get basic food supplies in Zimbabwe. Inflation has skyrocketed to more than 100,000 percent, and food production and agricultural exports have dropped drastically.
Once revered for providing some of the best education and health care in Africa, it now has one of the lowest life expectancies in the world. E-mail to a friend
CNN's Nkipele Mabuse in Johannesburg, South Africa, contributed to this report
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