Six people were arrested in Jakarta's satellite city of Bekasi, two others in northern Jakarta and another in western Jakarta, said National Police spokesman Dedi Prasetyo on Monday.

He said the group of nine, aged between 18 and 28 and led by a militant cell leader, Abu Zee Ghurobah, are believed to be linked to Jemaah Anshorut Daulah, a local militant network affiliated with the Islamic State group.

"They have been preparing themselves to attack police with military-style training in several places," Prasetyo said.

He said authorities defused a powerful bomb from one of the suspects, Muhammad Arshad, when they raided his house in northern Jakarta.

Separately, the North Jakarta police chief, Budhi Herdi Susianto, said they also found a goodbye letter written by Arshad, who allegedly planned an imminent suicide attack using the explosive at a police station.

Susianto said police seized a rifle, a gun, two knives, military-style uniforms, jihadi books and bomb devices from the suspects.

Prasetyo said police were questioning the group to determine whether they had links to extremist networks in Indonesia affiliated with the Islamic State group.

Indonesia, the world's most populous Muslim nation, has carried out a sustained crackdown on Islamic militants since bombings on the tourist island of Bali in 2002 killed 202 people, including 88 Australians.

The Jemaah Islamiah military network, which was blamed for the Bali attacks, was neutralised following the arrests of hundreds of its militants and leaders.

But new threats have emerged in recent times from Islamic State group-inspired radicals who have targeted security forces and local "infidels" instead of Westerners.

Australian Associated Press