Zimbabweans are voting in a presidential election that has already been hit by allegations of fraud.
On Tuesday, incumbent Robert Mugabe said he would resign after 33 years in power if he lost.
It came as Prime Minister Morgan Tsvangirai’s Movement for Democratic Change (MDC) accused Mr Mugabe’s Zanu-PF of doctoring the electoral roll. Zanu-PF denied the accusation.
Campaigning has been peaceful, with no reports of violence or intimidation.
The last presidential poll five years ago were overshadowed by myriad problems, including violence.
Voting began at 07:00 (05:00 GMT) and is scheduled to end at 19:00 (17:00 GMT), with results expected within five days.
The situation has been relatively calm ahead of the poll, with most bars in the capital Harare full on Tuesday night, given that Wednesday was declared a holiday to allow for voting, the BBC’s Brian Hungwe reports from Harare.
A large turn out is expected, given the tens of thousands of people who have gone to rallies staged by the candidates in recent weeks.
Zanu-PF responded to the allegations surrounding the electoral roll by saying it was the responsibility of the Zimbabwe Electoral Commission (Zec), which released the roll only on the eve of polls.
A Zanu-PF spokesman pointed out that appointees from both parties are on the commission and accused the MDC Finance Minister Tendai Biti of not funding Zec properly.
Zec has not commented on the allegations.
A BBC correspondent who has seen the electoral roll says it features the names of thousands of dead people.
Some names also appear twice or three times with variations to their ID numbers or home address.
Mr Mugabe and Mr Tsvangirai have been sharing power since 2009, under a deal brokered by the regional bloc to end conflict that marred elections in 2008.
At a press conference at State House in the capital, Harare, Mr Mugabe told journalists that he and Mr Tsvangirai had learnt to work together and could even share a pot of tea.
Responding to a question from the BBC, the president, who was in a jovial mood, said he would step down if he lost and insisted that there had been “no cheating”.
But the MDC has said the electoral roll released on Tuesday by Zec dates back to 1985 and is full of anomalies.
Three other candidates are also standing for president and voters will also be electing news members of parliament.
Bulawayo-based journalist Thabo Kunene told the BBC that many Zimbabweans had been returning home from South Africa to vote.
Taxis and buses carrying the exiles continued to arrive in the southern city on Tuesday afternoon, he said.
In Bulawayo’s oldest township of Makhokhoba, MDC and Zanu-PF campaign teams met amicably at one house during their door-to-door campaign – waving each other’s flags – a sign that some Zimbabweans had matured and no longer believed in violence, he added.
In 2008, Mr Tsvangirai pulled out of run-off vote, accusing pro-Mugabe militias and the security forces of attacking his supporters after he gained most votes in the first round.