President Abdulla Yameen has embraced China, while his opponent, Ibrahim Mohamed Solih, is seen as leaning towards India and the West.
The European Union and US have voiced concerns about the election.
Both have threatened to impose targeted sanctions if the democratic situation does not improve.
Police raided opposition headquarters on the eve of the vote, according to local reports.
Polls opened at 08:00 (03:00 GMT) on Sunday.
The Maldives is made up of 26 coral atolls and 1,192 islands, and tourism is a vital part of its economy. More than 400,000 people live there but its future hangs in the balance due to climate change.
What’s been happening?
The archipelago has been gripped by political upheaval in recent years. In February the Supreme Court quashed the convictions of nine opposition figures, among them exiled ex-President Mohamed Nasheed, who was ousted from office in 2012.
But after President Yameen declared a state of emergency and ordered the arrest of two judges, the court reversed its decision.
The move was seen as a sign that Mr Yameen – who is seeking to win another five-year term – would not tolerate any challenge to his rule and sparked criticism from Washington, London and New Delhi.
Some in India, meanwhile, called for an intervention in a small, neighbouring country once seen as firmly within its sphere of influence. Mr Nasheed also appealed for Indian military intervention.
What does China have to do with this?
As part of Beijing’s push to gain strategic influence and carve out new trading routes in the Indian Ocean and beyond, it has lent billions for huge infrastructural projects in Pakistan and Sri Lanka, and operates key ports in those countries, to the chagrin of India.
Under Mr Yameen, the Maldives has also welcomed Chinese money for major projects and signed a free trade agreement. More tourists from China now visit the Maldives than from any other country.
Analysts say that Beijing fears any change in government that could affect its interests, while India is concerned about Mr Yameen’s cosy ties with its regional rival.
“India sees it all in a very securitised sense that here are all these Chinese-backed ports around it,” said Gareth Price, a South Asia expert at the think-tank Chatham House, referring to Pakistan and Sri Lanka.
Delhi thus sees China’s lavishing of loans on the Maldives as part of a wider strategic ambition to secure another foothold in the Indian Ocean, he added.
Will the election be fair?
The opposition coalition says the Maldives election commission is working on behalf of Mr Yameen and will not allow observers to verify individual ballot papers, which could allow “massive vote counting fraud”.
Human Rights Watch has also criticised the actions of the government.
“Maldives authorities have detained critics, muzzled the media, and misused the election commission to obstruct opposition candidates to ensure President Yameen a victory on election day,” said the group’s Asia associate director Patricia Gossman.
On Saturday, police officers searched the opposition Maldivian Democratic Party office in Male without a warrant, the party said. A police spokesperson confirmed the raid to the BBC, without providing further details.
The commission’s spokesman has said allegations of possible voter fraud “don’t have any basis in reality” and that the vote counting process has not changed.
Mr Yameen has also shrugged off accusations of authoritarianism, saying during the campaign: “No-one will come to greet me and shake my hand, if there is tyranny.”
But in the lead-up to the vote his government has also been hit with fresh accusations of corruption. Journalists from the Organised Crime and Corruption Reporting Project said they had found that dozens of island leases had been given out to developers without public tender in 2014 and 2015, and alleged that Mr Yameen’s office “was recorded as intervening” in at least 24 deals.
Mr Yameen has denied the allegations.
His half-brother, Maumoon Abdul Gayoom, ran the country autocratically for three decades until the archipelago’s first-ever democratic vote in 2008, which was won by Mr Nasheed. Mr Gayoom was jailed in June.