Indonesian police Thursday offered prayers for peace and called for unity ahead of a massive protest by Muslim hardliners against Jakarta's governor that authorities fear could turn violent.

The much-hyped rally, planned for Friday, is expected to draw tens of thousands to the streets of the Indonesian capital in protest against governor Basuki Tjahaja Purnama, a Christian facing allegations of blasphemy.

The protest has authorities on edge, with police planning to deploy 18,000 officers across Jakarta as soldiers guard key landmarks and government buildings with armoured vehicles.

Some foreign embassies have warned their citizens to avoid the demonstration, which has seen organisers bus in hardliners from outside Jakarta and encourage solidarity marches in other major cities.

President Joko Widodo has met this week with religious and political leaders as well as with a former rival to issue a unified call against violence.

The protest was triggered by accusations Purnama, better known by his nickname Ahok, insulted Islam by criticising opponents who used Quranic references to attack him ahead of an election in February.

Hardliners have called for his death and predict Friday's turnout will eclipse a similar effort last month that drew 10,000 chanting demonstrators in white Islamic robes to the city centre.

Police have sought to soothe tensions ahead of the day, staging prayer sessions and erecting banners around the city declaring "we are all brothers".

At a mosque in Jakarta on Thursday, police wearing Islamic garb joined a prominent cleric in praying for a peaceful outcome.

"We pray that the demonstration will go smoothly, orderly and safely, and there will not be disruptions during the protest," said police spokesman Awi Setiyono.

Female police officers wearing hijab -- the traditional covering for the head and neck worn by many Muslim women -- will also be deployed to the frontlines "as negotiators", he added.

"They will distribute water and sweets for the protesters, to cool things down," Setiyono told AFP.

Indonesian military chief Gatot Nurmantyo encouraged people to go about their business as usual, but warned protesters they were ready if things got out of hand.

"If the worst case scenario happens, we will be saving the general public first, not violent protesters," he told reporters.

Purnama has faced constant opposition from hardline Islamic groups, who protested for weeks when he became governor two years ago.

Known for his tough-talking style, he told a crowd last month they'd been "deceived" by his opponents who had used a Quranic verse to try to put them off voting for a Christian.

Purnama became Jakarta governor in November 2014, but was not elected to the post. He was deputy governor and automatically became governor after incumbent Widodo was elected Indonesian president.