JAKARTA - Election rivals of Jakarta Governor Basuki Tjahaja Purnama joined thousands of Muslims gathered at the Masjid Istiqlal grand mosque on Saturday (Feb 11) as part of a veiled protest against him for insulting Islam, just four days before the most divisive gubernatorial elections in the capital to date.

The appearance of Gerindra Party pair of former education minister Anies Baswedan and businessman Sandiaga Uno, as well as Mr Agus Harimurti Yudhoyono - son of former Indonesian president Susilo Bambang Yudhoyono - and his running mate Sylviana Murni, came as a surprise to the crowd at the grand mosque.

The Forum of Muslims (FUI), a coalition of conservative Islamic groups, had organised the gathering during the first call to prayer on Saturday to urge Indonesians not to vote for a non-Muslim leader at the upcoming gubernatorial elections. Mr Basuki, popularly known as "Ahok", is Chinese Christian.

Many had arrived at the mosque in the heart of Jakarta as early as Friday midnight, but their numbers were nowhere near the hundreds of thousands that turned up during the previous anti-Ahok rallies held in the city last November and December.

Mr Basuki is running for re-election while being on trial for allegedly making an inappropriate reference to a Quranic verse at a grassroots event last September.

His remarks have led to an outcry by hardline Muslim groups in Indonesia and have threatened to turn the upcoming elections into a test of racial and religious tolerance in the world's most populous Muslim nation.

The FUI had initially planned to hold a street march from Masjid Istiqlal to the National Monument to mirror the anti-Ahok protest on Dec 4 organised by the National Fatwa Guardians of the Indonesia, and the hardline Front Pembela Islam (FPI), where more than 200,000 turned up.

But the police banned the FUI rally, saying it would be disruptive to hold a public rally five days before voters in Jakarta cast their ballots on Wednesday. The group then decided to get around the ban by calling for a mass prayer at the mosque as well as limiting any activity within the compound of the Istiqlal.

FPI leader Rizieq Shibab, who has been the fiercest critic of Mr Basuki, delivered a sermon on Saturday centred on the verse referenced by the governor in his allegedly blasphemous remarks.

Mr Rizieq himself, however, is also the subject of multiple police investigations for defaming founding president Sukarno, insulting the national ideology Pancasila and Christians, making hate speech as well as spreading pornography.

He had failed to report for police questioning twice this week, despite repeated summons issued to him but showed up at Istiqlal to deliver his sermon at about 8.35am local time, surrounded by bodyguards.

"I'm ready to be interrogated any time as long as the cases are not fabricated," said Mr Rizieq in his address to the crowd. "If the government treats (Islamic clerics) right, the people will also respect the government."

A downpour just an hour before the first call to prayers began at 4.30am, resulted in a low turnout earlier in the morning. But a convoy of private buses ferrying more people began arriving at the mosque just after 7.30am, filling up almost its entire compound.

Groups from as far as Semarang in Central Java, or Depok in West Java, were also spotted at Istiqlal mosque. Many could be heard chanting "Victory begins with pre-dawn prayers," or "God willing, Jakarta will get a leader who is soleh". Soleh refers to a Muslim who is deeply pious.

Mr Umah Hilmawan, a member of the Sunah Mustofa, a Muslim group from Depok, was at the Istiqlal at about 4.45am. He told The Straits Times that although he was not a resident of Jakarta, he supports Mr Agus and Ms Sylviana, and wants to appeal to Jakarta voters not to elect a non-Muslim governor. "We believe in Sylvi, the running mate of Agus, she will protect the Muslims' interest," he said.

"Are you all ready to vote for Muslim leaders? Are you all ready to save Jakarta?" Prosperous Justice Party (PKS) lawmaker Hidayat Nur Wahid asked the crowd during his speech at the mosque, to which they answered "yes".

Mr Basuki and his deputy Djarot Saiful Hidayat, who are backed by the ruling Indonesian Democratic Party-Struggle, are in a three-way contest to govern the country's capital. They are up against Mr Anies and Mr Sandiaga from Gerindra Party, and Mr Agus and Ms Sylviana from the Democratic Party.

The campaign will come to a close at midnight Saturday. Mr Basuki and Mr Djarot, who had taken leave to run for re-election, will return to work at City Hall on Sunday, the Ministry of Home Affairs confirmed on Friday night. This was despite calls for the governor to be suspended due to the ongoing blasphemy trial.

Electibility polls released over the last week suggest that the Basuki-Djarot pair remain the favourites, taking over mid-race leaders Agus-Sylviana, who had also lost ground to Anies-Sandiaga, particularly after the second debate organised by the Jakarta General Elections Commission.

A snap poll conducted by Jakarta Globe news on Twitter which asked users to vote for the best performer at the third and final debate on Friday night saw Mr Basuki and Mr Djarot scoring 78 per cent, Mr Anies and Mr Sandiaga 19 per cent and Mr Agus and Ms Sylviana 3 per cent.