"The opposition has won the election," Movement for Democratic Change secretary-general Tendai Biti said.

Based on the results of 243 constituencies posted outside polling stations, Biti said Tsvangirai won 50.3 percent of the vote -- just barely over the 50 percent margin to avoid a runoff with incumbent President Robert Mugabe.

But Zimbabwe's state-run newspaper announced Wednesday that neither party garnered more than 50 percent, and that a runoff was inevitable.

The government has yet to release any official results from the presidential vote, creating confusion.

Biti told CNN on Tuesday his party believed it had won enough votes to force a run-off election.

The MDC had announced on Sunday -- a day after the presidential vote -- that Tsvangirai had won 67 percent of the vote.

Harare blames the delay in releasing results on logistical reasons, noting that four elections were held simultaneously. But some election observers and analysts have raised concerns that Mugabe is using the time to rig the results.

The Zimbabwe Election Support Network, a group of non-governmental organizations monitoring the election, had released exit polling data that showed Tsvangirai leading with over 49 percent of the vote -- short of the 50 percent plus one vote needed to avoid a runoff election.

Mugabe was second with 41.8 percent, according to the organization. Independent candidate Simba Makoni had 8.2 percent.

The United States, Britain and the European Union have expressed their desire for Mugabe to step down.

U.S. Secretary of State Condoleezza Rice last weekend called Mugabe a "disgrace" to his country and the entire region.

The European Union said it was important for the Zimbabwe Electoral Commission to release the results and avoid "unnecessary speculation" about the results. British Prime Minister Gordon Brown said an immediate release of the results was "critical" so that the elections were seen to be fair.

Tensions are high in the southern African country that has had only one leader -- Mugabe -- since independence from Britain in 1980. There are fears that a delay in results could lead to violence.

A year after the last presidential election -- which the MDC said was stolen -- the government of Zimbabwe charged Tsvangirai for 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 imposed by the United States on Zimbabwean officials.

A hero of the country's civil war against the white Rhodesian government, Mugabe became the country's first black leader in 1980. Nearly three decades later, he has consolidated his rule over all aspects of Zimbabwean life.

His country was once revered for offering its citizens some of the best education and health care in Africa, but now schooling is a luxury and Zimbabwe has one of the lowest life expectancies in the world.

Zimbabwe was once known as the breadbasket of southern Africa, but now it is difficult to get even basic food supplies. Inflation has skyrocketed to more than 100,000 percent, while food production and agricultural exports have dropped drastically.

Thousands of Zimbabweans flood into neighboring countries to look for jobs.

Part of the economic freefall is traced to Mugabe's land redistribution policies, including his controversial seizure of commercially white-owned farms in 2000. Mugabe gave the land to black Zimbabweans he said were cheated under colonial rule, and white farmers who resisted were jailed.

Mugabe denies mismanagement and blames his country's woes on the West, saying sanctions have harmed the economy.