Madagascans are voting in a referendum on a controversial new constitution for the Indian Ocean island, which has been beset by instability for several years.
It would allow current leader Andry Rajoelina to stay in power as long as it takes to organise an election.
Mr Rajoelina, who seized power last year with the backing of the military, has said he will not run for president.
But all three of the main opposition groups, each led by an ex-president, are calling for a boycott.
They see the referendum as an attempt to legitimise Mr Rajoelina's position, which is only recognised by a few other countries.
Age limit change
BBC Africa analyst Mary Harper says although Mr Rajoelina has said he will not run for president, the new constitution would give him the option of changing his mind.
The new constitution would lower the age limit for presidential candidates from 40 to 35 years, allowing the 36-year-old to stand.
The former DJ, who ousted President Marc Ravalomanana in March 2009, organised huge rallies in support of a "yes" vote.
He also signed an agreement with 99 small political parties to hold elections next year.
Voting so far has been slow but peaceful in the capital Antananarivo, AFP news agency reports.
"I voted yes. I decided this will put a definitive end to this crisis," Tina Rakotovao, a doctor in Antananarivo, told Reuters news agency.
If many people observe the boycott, the result of Wednesday's referendum will be relatively meaningless.
Mr Rajoelina is diplomatically isolated, with few foreign powers recognising him as a legitimate head of state.
Madagascar, which is very poor, has also lost out on a lot of foreign aid since he took power.
It stands to lose a lot more if he continues to act alone, ignoring attempts by regional mediators to broker a consensus with the opposition, our analyst says.