People in Zimbabwe are voting 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 are on full alert amid fears of violence.
BBC Southern African correspondent Peter Biles says Mr Mugabe and Zanu-PF will be relying on support from voters in the countryside, while the MDC is strongest in the cities.
But in recent days it has become increasingly difficult to predict the outcome, he adds.
Mugabe's warning
Queues of voters had formed at some polling stations before the polls opened at 0500 GMT. Voting is due to end at 1700 GMT.
Nearly six million people are eligible to vote in the presidential, parliamentary and local elections being held on the same day.
On Friday, President Mugabe wrapped up his campaigning at a rally outside Harare with a fresh broadside against Britain and the MDC.
"This is a vote against the British," he told a crowd of 6,000 supporters, calling the MDC "a puppet, a mouthpiece of the British".
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 one adult in five is believed to have a regular job.
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.
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".
The chairman of Zimbabwe's electoral commission, George Chiweshe, has rejected the claim, saying the voters' roll was "credible".
The president has said the vote will be fair, warning opponents not to protest if they lost.
Increased military presence
Myjoyonline Ghana News Photos | Morgan Tsvangirai addressing his supporters
Morgan Tsvangirai addressing his supporters
On Friday, the chiefs of Zimbabwe's police, army, prison service and intelligence services warned that violence after the poll would not be tolerated.
Augustine Chihuri, commissioner general of the police, said: "The defence and security forces of Zimbabwe are on full alert from now onwards."
He said they would "thwart all threats to national security".
A BBC contributor in Masvingo, in south-eastern Zimbabwe, says there is an increased military presence and fly-pasts by fighter jets.
One villager told him: "We have been warned that if we vote Mugabe out, there will be war."
Another Masvingo resident said he would not be cowed by intimidation: "We do not mind even if they deploy soldiers at polling stations - it's time for Mugabe to go."
However, campaigning has been relatively peaceful, with none of the widespread intimidation of opposition activists seen in polls in recent years.
Both Mr Makoni and Mr Tsvangirai have been able to hold rallies across the country.
A total of 5.9 million people are eligible to vote in the elections.
A candidate must win more than 50% of the presidential vote to avoid a run-off in three weeks' time.
Source: MJFM
