Noel Kututwa, the head of the Zimbabwe Election Support Network, said late on Sunday: "The delay in announcing these results is fuelling speculation that there could be something going on."
The MDC says the government is sitting on the results in order to rig them in favour of Robert Mugabe, the president.
The opposition has made repeated claims that Mugabe has attempted to rig the vote to assure himself a sixth term in office and the MDC warned that it would not accept any declaration that Mugabe had won.
International observers say they are satisfied that the vote was fair, but some admit they are frustrated with the delay in official results.
Security forces were placed on high alert ahead of the election amid fears of a repeat of the bloodshed which followed Kenya's disputed elections last December.
Earlier, reporters mobbed Judge George Chiweshe, the electoral commission chairman, demanding to know when he would start announcing official results.
Chiweshe said it was taking time because Zimbabweans had voted for a president, the two houses of parliament and local councillors, so four ballots had to be counted for each voter instead of one.
"This has been a more complicated election. We will be releasing the results as soon as we can," he said.
Tendai Biti, secretary-general of the MDC, told a news conference early on Sunday: "This far, short of a miracle, we have won this election beyond any reasonable doubt."
Biti, who gave partial unofficial results based on returns posted at polling stations where counting had been completed, said: "In our view, this trend is irreversible."
Kamahl Santamaria, Al Jazeera's correspondent in Zimbabwe, said that electoral laws only allowed the Zimbabwe Electoral Commission (ZEC) to announce election results, a legal provision that the opposition has apparently ignored in saying that it had won 67 per cent of the vote.
Supporters of Morgan Tsvangirai, the MDC candidate, celebrated in the streets after Biti's announcement, giving each other the open-handed wave that is the opposition party's symbol.
In Harare's densely populated Mbare suburb, drivers hooted their horns as opposition supporters sang and danced.
Al Jazeera's Haru Mutasa reported that the opposition was basing its claim on results from urban areas, such as Bulawayo, where they have strong support.
|"It is called a coup d'etat and we all know how coups are handled" |
George Charamba, Mugabe spokesman warning the MDC
She said that in rural areas, where Mugabe's Zanu-PF is still popular, communication is not so good and that it would take several more hours before those results filter in.
George Charamba, Mugabe's spokesman, warned the MDC against an early victory claim.
"He announces results, declares himself and the MDC winner and then what? Declare himself president of Zimbabwe? It is called a coup d'etat and we all know how coups are handled," he told the state-run Sunday Mail newspaper.
Even before the polls closed on Saturday, Tsvangirai and foreign observers had cast doubts on the validity of the ballot, alleging that the electoral roll had been stuffed with phantom voters.
With the West having accused Mugabe, 84, of rigging the last presidential election in 2002, monitors from European Union countries and the United States were all excluded this time as were nearly all foreign media.
Mugabe has shrugged off any suggestion of vote-rigging, warning the MDC to respect any announcement on the result.
"Why should I cheat? The people are there supporting us day in, day out," he told Al Jazeera.
Once seen as the region's breadbasket, Zimbabwe is now suffering from previously of shortages of even the most basic foodstuffs, such as cooking oil and bread.
The country is grappling with the impact of the world's highest rate of inflation - officially put at 100,580.2 per cent - and an unemployment level which has reached the 80 per cent mark.
Mugabe has blamed the country's economic woes on the EU and the US, which imposed sanctions on his inner circle after he was accused of rigging his 2002 re-election.