A huge protest in Jakarta against Governor Basuki Tjahaja Purnama turned violent in the evening yesterday with some demonstrators burning police cars near the presidential palace.
Police said the protesters threw bottles and rocks, and security forces responded by firing tear gas and water cannon.
There was an unconfirmed report that one person was killed. Several policemen were also reported to have been injured.
Police said some 100,000 people had gathered earlier in the day to demand that Mr Basuki, better known as Ahok, either resign or be jailed for allegedly committing blasphemy while campaigning for re-election.
Waving Indonesian flags and singing the country's anthem, the sea of Muslim protesters led by hardline Muslim group Islamic Defenders Front took to the streets in Jakarta and cities including Bandung, Makassar and Medan.
The protest was sparked after Mr Basuki, a Christian from the ethnic Chinese minority, last month criticised opponents who cited a Quranic verse to attack him ahead of the gubernatorial election in February. He told Muslim constituents not to be deceived by his opponents who used the reference to discourage them from voting for non-Muslim leaders.
He had since apologised and clarified that he had no intention of insulting Islam, but that failed to quell anger among Muslim hardliners.
Coming from as far as Bali and Kalimantan, the Jakarta demonstrators first gathered at the country's biggest mosque, Istiqlal Mosque, to perform Friday prayers before marching 3.8km to the presidential palace, where they protested for hours until long after dusk.
Small pockets of protesters were calling for Mr Basuki's death. Student protesters in the South Sulawesi capital of Makassar had reportedly burnt tyres on the streets.
A protester in Jakarta, retired shopkeeper Dedi Hariyadi, 70, said Mr Basuki had misinterpreted the Quran, adding: "We are under threat because Ahok doesn't like Islam. But now he has committed slander, (he has) insulted Islamic clerics. We should speak up against it."
The police said they believed the demonstrators would include sympathisers of the Islamic State in Iraq and Syria (ISIS) from domestic militant groups such as Jemaah Ansharut Tauhid and Jemaah Ansharut Daulah, but did not expect them to commit violence.
A government source told The Straits Times that more than 20 Indonesians known to be ISIS sympathisers were spotted among the mob. One of them was militant Syamsudin Uba, long suspected to be the leader of a local terror cell with ties to the group in Syria.
There were also other members of a local group made up of Indonesians who used to fight alongside the mujahideen in Afghanistan during its struggle against the Soviets in the 1980s.
Indonesian Vice-President Jusuf Kalla met leaders of the Muslim hardliners behind the street marches, and promised that the authorities would expedite investigations against Mr Basuki. "In the matter concerning Ahok, we will carry out the legal process quickly and firmly," he told reporters. "Everything will be carried out in accordance with the law, in a firm manner."
Meanwhile, Mr Basuki remains the top favourite among Jakarta residents to manage the capital. He is widely seen as a clean and capable leader who has effectively tidied up the city, relocated squatters and rooted out incompetent civil servants. Jakarta residents told The Straits Times they were not bothered that he is ethnic Chinese and a minority Christian, and will vote for him in the February gubernatorial election.
The incumbent will be facing a three-way fight against former education minister Anies Baswedan and former president Susilo Bambang Yudhoyono's son, Mr Agus Harimurti Yudhoyono.