Indonesian police have arrested a teenager after he attempted to attack a church in Medan, the capital of North Sumatra, with a homemade bomb.

The 17-year-old was sitting among the morning congregation at St Yoseph Catholic Church yesterday. He had with him a small backpack purportedly containing a small explosive device, North Sumatra provincial police spokesman Rina Sari Ginting told reporters.

According to Ms Nana Manullang, who was sitting next to him, the teenager had been "assembling a suspected bomb", comprising a battery and a pipe hidden in a gold jacket before the attack.

He then stood up, with his backpack giving off "smoke, sparks and faint sounds of firecrackers", and rushed towards the priest near the pulpit, attacking the latter with a knife. The priest was slightly wounded on his left arm. Members of the congregation then restrained him, she added.

National police spokesman Boy Rafli Amar confirmed the attack. He added that the attacker was still alive and there were no casualties.

Police spokesman Agus Rianto said "for certain, there was an explosion like firecrackers".

According to a source close to the investigation, police confiscated several items, including a backpack, a knife, and a note with "La ilaha illallah" in Arabic written on it. It is a declaration of the Islamic faith which means "there is no God but Allah".

The identity card on the teenager stated his name as Ivan Armadi Hasugian, a student from Medan.

Police have identified the pastor as 60-year-old Albert Pandiangan.

Indonesia has been the target of several terror threats in recent months, and counter-terrorism police have launched a crackdown on people with suspected links to the Islamic State in Iraq and Syria (ISIS) group .

A Jan 14 terrorist attack in Jakarta killed eight people, and a July 5 suicide attack near a local police station in Solo city killed the bomber and injured a policeman.