Voters in Jakarta are choosing a governor in a run-off election, that has been called "the dirtiest and most divisive" in Indonesia.

Polls suggest it is a close race between incumbent Basuki Tjahaja Purnama, a Christian, and Anies Rasyid Baswedan, a Muslim.

Security is tight in the Indonesian capital amid heightened racial and religious tensions.

Mr Purnama won the first round, but not by enough of a margin.

BBC Indonesian editor Rebecca Henschke says that though extra security officers have been deployed to polling stations, the mood in the capital is not overly tense and there is still a festive atmosphere.

The tensions were sparked by a controversy surrounding Mr Purnama, who is accused of mocking a Koranic verse for which he is on trial for blasphemy. He has denied the charges.

Mr Purnama, also popularly known as "Ahok", voted with his family at a polling station in north Jakarta early on Wednesday morning.

He told reporters: "Jakartans must use their voice as the future of Jakarta is in their hands. Don't be afraid, the police are here providing security."

A coalition of hardline Islamic groups supporting Mr Basedan said it was deploying at least 100 monitors at each polling station across the capital.

Rizieq Shihab, leader of the Islamic Defenders Front which has been leading protests against Mr Purnama, has also cast his vote.

Asked by the BBC if his group was damaging Indonesia's pluralist democracy, he said: "Democracy doesn't stop someone from voting for a person from the same religion as you.... Christian vote for Christian, Muslims vote for Muslim."

What is the controversy about?

Mr Purnama was accused last year of insulting a Koranic verse during a campaign speech.

The verse had been used by his opponents in the world's most-populous Muslim country to argue that Muslims should not vote for a non-Muslim leader.

But Mr Purnama denies blasphemy, saying his comments were aimed at politicians "incorrectly" using the Koran against him.

If convicted, he faces a maximum five-year jail sentence, though could still govern while appeals are heard. His trial is due to resume on Thursday.

Hundreds of thousands have taken part in protest rallies led by Islamist groups against Mr Purnama in recent months.

The controversy has clearly hurt Mr Purnama's chances. Once considered the clear frontrunner, he won the first round of the election in February with only 43% of the vote, while Mr Baswedan had 40%.

Mr Purnama is the first Christian and minority ethnic Chinese leader of Jakarta in over 50 years.

He stepped into the role from the deputy post without election in 2014, when his predecessor, Mr Joko Widodo, became president.

How has it affected the country?

The Jakarta Post has described the campaign as "the dirtiest, most polarising and most divisive the nation has ever seen".

This election is being seen as a choice between the secular policies Indonesia has practised since independence and a hardline political Islam that has strengthened in recent years, our correspondent says.

About 85% of Indonesia's population are Muslim, but the country officially respects six religions.

The election has also seen anti-Chinese sentiment, sparking unease in a country that has seen violence against its Chinese minority previously.