CILACAP, INDONESIA (AFP) - Hundreds of radical supporters of a jailed Indonesian cleric chanted "God is great!" outside court on Tuesday (Jan 26) as the firebrand preacher tried to overturn his conviction for funding a militant group.

Just 12 days after a deadly gun and suicide bomb attack in Jakarta, security was intensified as Abu Bakar Bashir returned to the Central Java court to challenge his 15-year sentence for helping fund a paramilitary group in Indonesia's far west.

Around 1,500 police and military personnel were deployed to secure the streets around the courthouse, far more than at Bashir's previous hearing which took place just two days before the Jan 14 attack in the Indonesian capital.

Bashir is seeking a judicial review of the conviction passed in 2011.

Wearing a flowing white robe, spectacles and turban, Bashir spoke little during Tuesday's hearing but his supporters were vocal, shouting at the judge and prosecutors.

"Free Bashir, he is not a terrorist!" one cried.

Bashir's legal team argues that funds he collected were intended to help people in the Palestinian territories, but ended up getting sent to a militant group in Aceh without his knowledge.

The 77-year-old is regarded as a spiritual leader of militant Islam in Indonesia. He was thought to be a key inspirational figure in regional terror network Jemaah Islamiyah, blamed for the 2002 Bali bombings that killed more than 200 people.

Though his influence has waned, hundreds of his followers - along with supporters of the hardline Islamic Defenders Front (FPI) - packed the court and spilt onto the forecourt, where police kept watch with a mobile water cannon.

"I have been here since yesterday to support Bashir and Habib Rizieq," said one outside court, the latter a reference to the FPI founder who gave evidence Tuesday.

Bashir has distanced himself from the Jakarta attacks. Police blame a local affiliate of the Islamic State in Iraq and Syria for the assault that left four militants and four civilians dead.