Update: the polls are now closed, with the Movement for Democratic Change confident of victory, even amid growing reports of fraud, including instances of inflated voter rolls loaded with fictional names.
Update: Voting is underway now, with tales of long lines and "irregularities", including police harassment of voters coming back over the border from South Africa. Jordan's updating the story above.
Polling starts in a few hours, at 0500 GMT, and closes at 1700 GMT. Current president Robert Mugabe's competitors say they expect an unfair election, a claim denied by the government.
Zimbabwe has the world's highest inflation rate, and only one in five are believed to have jobs.
People in Zimbabwe are due to vote in an election that will decide whether President Robert Mugabe, in power since 1980, wins a sixth term in office.
His challengers are Morgan Tsvangirai of the MDC opposition party and Simba Makoni, a defector from Mr Mugabe's Zanu-PF now standing as a independent.
On the eve of the election, the president said the being a puppet of the former colonial power, Britain.
The MDC says the poll will be rigged - a charge denied by the government.
Zimbabwe's security forces have been put on full alert amid fears of violence after the election.
Polling stations across Zimbabwe are due to open at 0500 GMT and close at 1700 GMT. Nearly six million voters are eligible to cast their ballots.
MDC leader Morgan Tsvangirai and former Finance Minister Simba Makoni are President Mugabe's main challengers.
Mr Tsvangirai claims to have made inroads into the president's traditional support base in the countryside.
Meanwhile, Mr Makoni could attract many in the Zanu-PF party who have become disillusioned with Mr Mugabe, BBC Southern Africa correspondent Peter Biles says.
On Thursday, Mr Tsvangirai and Mr Makoni jointly expressed severe concerns about the polls.
In a statement, they said they had still not received full nationwide voters' lists that could be verified, and suspected there were many thousands of "ghost voters".
Mr Mugabe's challengers have also complained that there were not enough polling stations in urban areas and no equal access to media.
The president has said the vote will be fair, warning opponents not to protest if they lost.
A total of 5.9 million people are eligible to vote in the joint local, senate, assembly and presidential polls.
A candidate must win more than 50% of the presidential vote to avoid a run-off in three weeks' time.