Zimbabwe's opposition leader, Morgan Tsvangirai, has said that some people are being forced to vote in the run-off presidential election.
Mr Tsvangirai told reporters that people were being intimidated to vote but he said millions were refusing to do so.
Meanwhile, the head of an African observer mission in Zimbabwe has said polling in the country's presidential run-off is set to be very low.
President Robert Mugabe is certain to be elected as he is the only candidate, although the name of opposition leader Morgan Tsvangarai remains on the ballot.
The Movement for Democratic Change leader said the election, which he decided to boycott after a wave of deadly attacks against his supporters, was a source of shame.
'Today is not an election. Today is a shameful humiliation, another tragic day in our nation's history,' he wrote in a letter to supporters.
He said today's results will be meaningless because they do not reflect the will of the people of Zimbabwe. He said the results will only reflect the fear of the people of Zimbabwe.
Mr Tsvangirai asked people not to vote, if possible. But he said those who felt threatened should vote for Mr Mugabe.
A total of 5.9m Zimbabweans are theoretically entitled to cast their ballots in today's election, which is being overseen by some African but no Western monitors.
Voting at a Harare polling station earlier, Mr Mugabe said he felt very optimistic. Yesterday, he rejected international calls for the election to be postponed or called off.
Widespread criticism
There has been major international criticism of Mr Mugabe and his government.
The Minister for Foreign Affairs, Micheál Martin, has condemned the conditions under which the election takes place and has stated that the results of the vote could not be regarded as legitimate.
The European Commission called the presidential election a sham whose result will be 'hollow and meaningless'.
G8 foreign ministers have said they will not accept Zimbabwe's government as legitimate if it 'does not reflect the will' of the people.
Britain, Canada, France, Germany, Italy, Japan, Russia and the US said they deplored Zimbabwean authorities' systematic violence, obstruction and intimidation.
They said the results of the first round of voting on 29 March, in which Mr Tsvangirai topped Mr Mugabe but fell short of a majority, must be respected.
US Secretary of State Condoleezza Rice told a joint press conference that 'this kind of sham cannot produce a legitimate outcome'.
British Foreign Secretary David Miliband said the election was one-sided in every aspect.
'One-sided in brutality of the regime, one-sided in the publicity, even one-sided in electoral organisation,' he said.
Italy's Foreign Minister Franco Frattini said his government wants EU foreign ministers to consider withdrawing the union's ambassadors from Harare.
The UN Security Council is to discuss more sanctions against Zimbabwe next week after today's widely condemned vote.
