Zimbabwe's election was a "huge farce", Prime Minister Morgan Tsvangirai has said, alleging vote-rigging by rival President Robert Mugabe's camp.
Mr Tsvangirai said Wednesday's presidential poll was "null and void".
The largest group of election observers described the vote as "seriously compromised" and said up to a million Zimbabweans were prevented from voting.
Meanwhile, first official results from national assembly elections show Mr Mugabe's party taking an early lead.
Zanu-PF won 25 of 28 seats announced, although they were mostly in Mr Mugabe's rural strongholds, correspondents say.
Mr Mugabe's party, which is claiming a victory, has denied accusations of vote-rigging, saying the voting went smoothly.
The 89-year-old president is running for a seventh term.
Voters were choosing a president, 210 lawmakers and local councillors and the Zimbabwe Electoral Commission (ZEC) has five days to declare who won the poll.
Extra police units, some in riot gear, have been deployed in the capital, Harare.
Legal challenges are now likely to follow, but much will depend on whether Zimbabwe's neighbours endorse the poll, says the BBC's Andrew Harding.
Speaking at the headquarters of his Movement for Democratic Change (MDC) party in Harare, Mr Tsvangirai said: "Our conclusion is that this has been a huge farce.
"The credibility of this election has been marred by administrative and legal violations which affected the legitimacy of its outcome.
"It's a sham election that does not reflect the will of the people."
Some international observers have praised the conduct of the election.
But the largest group of domestic observers, the Zimbabwe Election Support Network (ZESN), said the voting was compromised.
ZESN chairman Solomon Zwana told the BBC the organisation remained "concerned about some cases of irregularities that have occurred in some areas".
The ZESN said potential voters were much more likely to be turned away from polling stations in urban areas, where support for Mr Tsvangirai is strong, than in President Mugabe's rural strongholds.
It claims that up to a million of the country's 6.4 million eligible voters were prevented from casting their ballots.
'Free, fair and credible'
Zanu-PF's senior members were already celebrating victory.
"We have romped [to victory] in a very emphatic manner," one member, who asked not to be named, told AFP news agency.
Zanu-PF's information secretary, Rugare Gumbo, said observers from several countries were all satisfied with the conduct of the election.
"I don't understand where this thing of saying the election was a farce is coming from," he told the BBC.
Britain said it was too early to comment on the outcome of the election, but noted the "peaceful atmosphere".
A spokesperson for the Foreign Office said it was "concerned by the late publication of the electoral roll and reports of large numbers of voters who were turned away".
If no candidate in the presidential vote gains 50% of the ballots, a run-off will be held on 11 September.
Zanu-PF and the MDC have shared an uneasy coalition government since 2009 under a deal brokered to end the deadly violence that erupted after a disputed presidential poll the previous year.