(CNN) -- Election observers called on Zimbabwe's government to release the results of weekend elections as soon as possible to avoid political unrest, but government officials said the results won't be out until Monday.
Zimbabwean President Robert Mugabe campaigns in the capital of Harare last week.
Justice George Chiweshe, the chairman of the Zimbabwe Electoral Commission, announced the commission would begin releasing results at 6 a.m. Monday (midnight Sunday ET).
Appearing on Zimbabwe's ZTV television network, he said commissioners had to verify results and would not be pressured into releasing them early.
"We are dealing with a matter with national dimensions ... Our mandate is clearly laid down in terms of the Constitution and we have tried -- actually, have followed -- that," Chiweshe said.
But Marwick Khumalo, a spokesman for the Pan-African Parliament observer mission, said he had no doubt the election commission knew "at least a larger part, if not all the results," by Sunday evening.
"So really, it is frustrating not only for the Zimbabweans themselves, even for those of us who come from afar, to come and witness this historical event," Khumalo said.
Saturday's vote posed a serious challenge to President Robert Mugabe's three-decade rule over Zimbabwe. Former finance minister Simba Makoni, and Morgan Tsvangirai of the main opposition party, Movement for Democratic Change, were vying to unseat the 84-year-old incumbent.
Defying a government order, MDC released its count of the votes Sunday and claimed an early victory for Tsvangirai. The party said it tallied the results posted outside each polling station -- and based on one-third of the returns, that count showed Tsvangirai won 67 percent of the votes, journalists inside Zimbabwe told CNN. Watch polls close in Zimbabwe »
The Zimbabwean government has denied CNN and other international news organizations permission to enter the country to report on the elections.
At its Sunday news conference, MDC also claimed it has won the majority of parliamentary seats in Zimbabwe's urban centers, including Harare and Bulawayo. MDC enjoys widespread support in the cities. Mugabe's ruling Zanu-PF party has its base in Zimbabwe's rural areas.
Takura Zhangazha, a local media watchdog representative, said it is unclear if the government would actually release results Monday.
Speaking to CNN from Harare, Zhangazha warned the government needs to announce the results "as soon as possible to allay any fears of rigging or cheating in terms of the vote."
"There's too much speculation, and the speculation tends to lead to a lot of political tension," said Zhangazha, part of the Media Institute for Southern Africa.
"I would not want to see people then fighting over the results or any political violence or tensions then occurring."
MDC leaders have already dismissed the government's unannounced results as rigged in favor of the incumbent leader and began declaring victory just hours into Saturday's vote. But there are concerns that if each side claims victory, tensions could ignite and violence could erupt.
Simba Makoni, who was expelled from the Zanu-PF after announcing his bid to unseat Mugabe, said it was "premature to judge that the environment before the balloting has had some impediment."
"Overall it was not a conducive environment" for voting, Makoni said, citing voter confusion and lack of access to the media. "But we know our people are clear about what they want and we expect they will express their will. We will wait and see the results."
Critics of the government predicted the vote would be rigged or marred by fraud. The United States this week warned of a possible unfair election, and New York-based Human Rights Watch announced earlier this month that the elections were likely to be "deeply flawed."
A hero of the country's civil war against the white Rhodesian government, Mugabe became the country's first black prime minister in 1980. But 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.
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 who he said were cheated under colonialist rule, and white farmers who resisted were jailed.
In 2005, Mugabe launched Operation Clean Out the Trash, in which he razed slum areas across the country.
Mugabe denies mismanagement and blames his country's woes on the West, saying sanctions have harmed the economy. E-mail to a friend
CNN's Robyn Curnow in Beitbridge, South Africa, and Eunice Mafundikwa contributed to this report.