Zimbabwe's presidential election was a "huge farce", PM Morgan Tsvangirai has said, alleging vote-rigging by rival President Robert Mugabe's camp.
At a news conference, Mr Tsvangirai said that Wednesday's poll was "null and void".
The largest observer group earlier said up to a million people were prevented from voting.
Mr Mugabe's party - which is claiming a victory - denied the accusations, saying the voting went smoothly.
The head of the African Union observer mission, former Nigerian President Olusegun Obasanjo, said his initial assessment was that the vote was free and fair.
Other regional observers praised the peaceful nature of the election.
Vote counting started overnight, and the Zimbabwe Electoral Commission (ZEC) has five days to declare who won the poll.
Police have warned they would take action against anyone trying to leak early results. It is illegal to publish unofficial results.
Extra units - some in riot gear - have now been deployed in the capital, Harare.
Legal challengers are now likely to follow, the BBC's Andrew Harding in Johannesburg reports.
But much will depend on whether Zimbabwe's neighbours endorse the poll, our correspondent adds.
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