Vote counting is under way in Zimbabwe, with the main opposition MDC claiming it is winning the battle to oust President Robert Mugabe.
The MDC said it was ahead in most constituencies but continues to fear the vote will be rigged.
Results may not be finalised for some days and the government warned the MDC not to declare an early victory.
Mr Mugabe is battling the MDC's Morgan Tsvangirai and independent Simba Makoni for president.
The Movement for Democratic Change (MDC) said early results showed it had won the poll.
"We have won this election," said its secretary general Tendai Biti early on Sunday. "This trend is irreversible."
He said early results posted at polling stations showed Mr Tsvangirai would win 66% in Harare.
But BBC Southern Africa correspondent Peter Biles says the MDC is working on the basis of partial, unofficial results.
President Mugabe and his ruling Zanu PF have always had majority support in the countryside, our correspondent says, and the rural areas may decide the outcome of the elections.
Mr Biti also claimed the MDC was making inroads in the rural areas, including Mr Mugabe's home province of Mashonaland West.
Zimbabwean Information Minister Sikhanyiso Ndlovu accused Mr Biti and the MDC of "speculation and lies" and "causing unnecessary havoc".
The state-run Sunday Mail quoted the ministry's secretary, George Charamba, as saying that if Mr Tsvangirai declared himself president "it is called a coup d'etat and we all know how coups are handled".
Mr Biti said the MDC was just "protecting its vote" and would not make the "mistake" of the 2002 and 2005 elections when it did not claim victory.
Before the election, state-run media predicted Mr Mugabe would win 57%.
A candidate needs 51% in the presidential vote to avoid a run-off.
The MDC has repeatedly expressed fears the election would be rigged.
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.
A presidential decree ahead of the elections had permitted police to go into polling stations, ostensibly to help illiterate voters.
The opposition feared many voters would be intimidated and stay at home.
Pan-African Parliament observers have also written to the electoral commission, saying that more than 8,450 voters had been registered on a patch of deserted land in Harare.
Most Western observers were banned from the election.
Some voters told the BBC they had been turned away but others that the system had worked efficiently and the atmosphere was good.
After voting in Harare, Mr Mugabe dismissed concerns of vote-rigging, saying: "We don't rig elections. I cannot sleep with my conscience if I have rigged."
President Mugabe, in power since 1980 and seeking a sixth term, said of his chances: "This time around, like the last time, very good... we will succeed and we will conquer."
Of the possibility of a second round, he said: "We are not used to boxing matches where we go from round one to round two. We just knock each other out."
Mr Mugabe's rivals were also confident.
Mr Tsvangirai said: "Victory is assured in spite of the regime's attempt to subvert the will of the people."
HAVE YOUR SAY
Mr Makoni said: "I feel good, I voted for the best candidate, I voted for Simba Makoni."
The elections will no doubt be free and fair. The allegations being peddled are unfounded and based on hearsay
Sosten Musiniwa, Harare
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.
The chiefs of Zimbabwe's police, army prison service and intelligence services warned on Friday that violence after the polls would not be tolerated.