At least 200,000 conservative Muslims have gathered for Friday prayers and to protest against Jakarta's governor, at a rally in the Indonesian capital.
Basuki Tjahaja Purnama, better known as "Ahok", is accused of insulting Islam during election campaigning.
On Wednesday, Indonesian prosecutors confirmed his case could go to trial.
Mr Purnama is Christian and ethnic Chinese - a double-minority in Muslim-majority Indonesia, where ethnic Chinese are about 1% of the population.
In a surprise move, President Joko Widodo, a political ally of Mr Purnama, appeared at the gathering and listened to a sermon delivered by one of his own fierce critics, radical Islamic Defender Front (FPI) leader Rizieq Shihab.
Ahead of the rally, police announced that they had detained eight people for suspected treason. The group includes the sister of former president Megawati Sukarnoputri and musician Ahmad Dhani.
Mr Widodo has blamed "political actors" of trying to take advantage of the furore over Mr Purnama to destabilise his government.
The BBC's Ging Ginanjar, at the rally, says organisers told police it would just be a public sermon and mass prayer, but many attendees are carrying banners condemning Jakarta's governor and calling for him to be jailed.
A large protest against the governor on 4 November turned violent, leaving one man dead and dozens of police and demonstrators injured.
In a campaign speech in September, Mr Purnama said Islamic groups who were using a Koranic verse to discourage support for him were deceiving voters. The verse is interpreted by some as prohibiting Muslims from living under the leadership of a non-Muslim.
Islamic groups said he had criticised the Koran and lodged complaints with the police.
Mr Purnama later apologised but denied committing blasphemy, which carries a maximum five year jail sentence. He has promised to continue campaigning for the governorship, a role he inherited when his predecessor Joko Widodo became president in 2014.
The election is to be held in February.
Police have said they will not arrest the governor, but he is barred from leaving the country while the case continues.
While polls suggest the straight-talking governor's popularity has been hurt by the allegations, he is popular for his stances against corruption and in favour of public transport and greater access to healthcare and education.