•Political tensions have been running high, with the opposition saying that the incumbent, Andry Rajoelina, should be disqualified.
•Opposition leaders have complained of an "institutional coup" in favour of Mr Rajoelina.
Madagascar is holding presidential elections despite an opposition boycott following weeks of protests.
A curfew was declared overnight to prevent any trouble.
Political tensions have been running high, with the opposition saying that the incumbent, Andry Rajoelina, should be disqualified.
There have been six weeks of demonstrations, which have seen opposition supporters clash with police.
Angelo Ravelonarivo, a police prefect in the capital of Antananarivo, said the curfew was imposed following "various acts of sabotage" including the "burning of a polling station".
Opposition leaders have complained of an "institutional coup" in favour of Mr Rajoelina.
They say he should be disqualified because he acquired French nationality in 2014.
Ten out of 12 opposition candidates have raised concerns about the vote's credibility and called on voters to boycott the elections.
Opposition supporters have been using cauliflowers as a symbol of police repression.
Former President Marc Ravalomanana, ousted by Mr Rajoelina in a 2009 coup, was seen holding one as he joined calls for a boycott.
He said: "If this election is not postponed, Madagascar will experience a major crisis because the population will reject the results."
Roland Ratsiraka, one of the protesting candidates, described the elections to AFP as a "fraud" and a "joke on Madagascar."
A Red Cross worker helping in the capital told the BBC: "The situation is intense every day. We have many injuries and we've had to provide first aid to 36 people."
According to the World Bank, Madagascar is one of the poorest countries in the world, with 75% of people living below the national poverty line in 2022.
The country has reserves of cobalt, gold, nickel, uranium and other minerals. However it has struggled with sluggish growth and persistent poverty for decades.
So whoever is going to take over will be expected to address widespread poverty, among others challenges.
With a population of around 30m, just 11m people are registered to vote in Madagascar.
Polls open at 06:00 local time (03:00 GMT) and close at 17:00 (14:00 GMT) on Thursday.
Mr Rajoelina, who officially resigned in September in order to run for re-election, has denied the criticism from opposition leaders and expressed confidence that he will win.
According to AFP, his government has said the protests are motivated by a "desire to overthrow power" and to "sabotage the electoral process".
It has also accused the opposition of "threatening the stability of the country".
Originally the president of the Senate was supposed to take over from Mr Rajoelina but declined. This left the decision to a "collegial government" headed by the prime minister, seen as a Rajoelina ally.
Despite this being accepted by the Constitutional Court, many are angered by Mr Rajoelina's candidacy because of his dual French nationality.
After taking power in 2009, Mr Rajoelina became Africa's youngest head of state.
He did not contest the 2013 election but was voted back into power in 2018.