A would-be suicide bomber's explosives failed to detonate in a packed church in western Indonesia during Mass on Sunday.
Police said he injured a priest with an axe before being restrained.
The 18-year-old assailant left a pew and ran toward the priest at the altar, but a bomb in his backpack only burned without exploding, said national police spokesman Maj Gen Boy Rafli Amar.
Before he was restrained by members of the congregation, the man managed to take an axe from the backpack and attacked Fr Albert Pandiangan, causing a slight injury to the 60-year-old priest's hand.
The motive for the attack at St Yoseph Church in Medan, the capital of North Sumatra province, was not clear, but the perpetrator carried a symbol indicating support for ISIS.
Police were interrogating the man, who told them he was not working alone, Amar said, without providing details.
Indonesia, the world's most populous Muslim nation, has carried out a sustained crackdown on militant networks since the 2002 Bali bombings that killed 202 people.
