Election observers in Zimbabwe have expressed concerns over "delays" in announcing official presidential poll results, amid fears of rigging.
The head of one monitors' group said he had "no doubt" officials now know most results. None have yet been released.
The election commission said late on Sunday that no results would be announced until 0400GMT on Monday.
Riot police have been patrolling the streets of the capital, Harare, and residents were told to stay indoors.
Saturday's election pitted the MDC's Morgan Tsvangirai and the independent Simba Makoni against President Robert Mugabe, Zimbabwe's leader since 1980.
House of Assembly, Senate and local elections were held on the same day, and officials say that this is why results have been slow to come.
Twenty-four hours after the polls closed, MDC supporters were celebrating in some areas after the party's leaders claimed to have won 67% of the vote, on the basis of what they say are returns from one-third of polling stations.
The government and election officials issued warnings to the MDC after it claimed victory, on the basis of the partial, unofficial results.
Information Minister Sikhanyiso Ndlovu accused the MDC of "speculation and lies" and "causing unnecessary havoc".
The first official results had been widely expected on Sunday.
"The delay in announcing these results is fuelling speculation that there could be something going on," said Noel Kututwa, the head of the Zimbabwe Election Support Network (ZESN).
"I'm worried," said Marwick Khumalo, head of the Pan-African Parliamentary Observer Mission, "these are the delays that start causing problems in a country".
Results were being posted outside individual polling stations and then collated from across the country by the electoral commission.
Poll observers from the Southern African Development Community said the elections had been "peaceful and credible", although one of two dissenting members of the mission said the polls were "deeply flawed".
Most Western observers were banned from the election.
"We have won this election," said the secretary general of the Movement for Democratic Change (MDC), Tendai Biti. "This trend is irreversible."
The party had won most parliamentary seats in Harare and Bulawayo and was also ahead in some rural areas, including Mr Mugabe's home province of Mashonaland West, he said.
BBC Southern Africa correspondent Peter Biles says the MDC's claims are based on partial, unofficial results.
The more slowly-counted votes from the rural areas, where President Mugabe has always had majority support, may decide the final outcome.
BBC contributors say opposition activists have been celebrating in the towns of Bulawayo, Mutare and Masvingo.
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A spokesman for Mr Makoni told the BBC News website that the MDC had "swept the board" in the parliamentary election, with several ministers losing their seats.
The elections will no doubt be free and fair. The allegations being peddled are unfounded
Sosten Musiniwa, Harare
But Zimbabwe's chief elections officer expressed concern "that some stakeholders have gone on to announce purported results... when in fact the results are being verified and collated".
The MDC says the commission was appointed by Mr Mugabe and is not to be trusted.
A British Foreign Office minister, Mark Malloch-Brown, said it was "quite likely" that President Robert Mugabe had lost the election in Zimbabwe, despite "massive pre-election day cheating".
A candidate needs more than 50% in the presidential vote to avoid a run-off in three weeks' time.
Across the country on Saturday, there were reports of voters not being allowed to cast ballots - either because their names were not on the voters' roll or because they were trying to vote in the wrong ward.
But many voters told the BBC the system had worked efficiently and the atmosphere was good.
After voting in Harare, Mr Mugabe, who is seeking a sixth term, said: "We don't rig elections. I cannot sleep with my conscience if I have rigged."
The MDC says it is fighting to save Zimbabwe's economy.
The country has the world's highest inflation rate, at more than 100,000%, and just one adult in five is believed to have a regular job.