Zimbabweans are voting in a key referendum on a new constitution, amid simmering political tensions.
Both main parties - Zanu-PF and the MDC - are urging their supporters to back the constitution, which would pave the way for elections later this year.
The two parties are in a shaky power-sharing deal, and there was some violence ahead of the referendum.
The draft constitution limits the president to two five-year terms - but this will not apply retrospectively.
That means 89-year-old President Robert Mugabe - who has led Zimbabwe since independence in 1980 - can stand for re-election in a poll expected in July.
Mr Mugabe's Zanu-PF party and the Movement for Democratic Change (MDC) led by Prime Minister Morgan Tsvangirai both back the new constitution, meaning it is almost certain to be approved.
The two parties agreed to share power in 2008 to resolve a violent dispute over the result of Zimbabwe's last elections.
Mr Mugabe and Mr Tsvangirai, 61, are both expected to stand for president again.
As he cast his vote, Mr Mugabe told the BBC that Zimbabweans had been widely consulted about the constitutional changes, though he acknowledged they may have needed more time to consider the final document.
"The views and the ideas came from the people so we can't be blamed for having ignored the people," he said.
Mr Mugabe also urged Zimbabweans to ensure the referendum went ahead peacefully.
The draft constitution is seen as a compromise document, analysts say, but political tension is nonetheless high.
Campaigning was marred by an attack on an MDC politician in the capital, Harare.
Sten Zvorwadza, who hopes to become the next Movement for Democratic Change (MDC) lawmaker for the city's Mbare suburb, was punched as he tried to put up posters on Friday.
The BBC's Andrew Harding witnessed the incident in Mbare - an MDC stronghold - and said the youths also threw punches at him and his film crew.
Although no-one was seriously injured, the incident is a reminder of the violence lurking close to the surface in Zimbabwe, our correspondent says.
On Tuesday, a Zanu-PF official was injured after his house was petrol-bombed by unknown assailants in Makoni district, in north-eastern Zimbabwe.
And in February, the 12-year-old son of an MDC activist died in an alleged arson attack in the eastern farming district of Headlands.
Polls across Zimbabwe opened at 05:00 GMT and are due to close at 17:00 GMT.
During campaigning the MDC has focussed on clauses guaranteeing protection against all forms of violence, and free and fair voting.
The MDC's Tendai Biti, who currently serves as finance minister, told the BBC the document was the "midwife" to a brand new Zimbabwe as it sets out people's rights, devolves some power and sets up a system of check and balances for those in authority.
The Zanu-PF campaign has highlighted the irreversibility of the land reform programme, which saw some 4,500 farms seized from mainly white commercial farmers, and other moves intended to give more economic power to black Zimbabweans.