A would-be suicide bomber attacked a Catholic priest after his explosives failed to detonate during Sunday Mass at a packed church in western Indonesia, police said.  

The attack took place at the Roman Catholic St. Joseph Church in Medan, the capital of North Sumatra province. City police spokeswoman Rina Sari Ginting described the failed attack as "terrorism" in a statement to Reuters. 

The assailant was sitting with other worshippers when he left a bench and ran toward the priest at the altar, but the bomb in his backpack did not go off, said national police spokesman Maj. Gen. Boy Rafli Amar.

"There was a small explosion like fireworks and he also took out a knife as he ran toward the priest," witness Timbas Ginting told Reuters. 

The man kept running toward the priest who was giving a sermon with a burning backpack, as the shocked congregation chased and captured him.

Before he was restrained, he managed to take an ax from the backpack and attacked the priest, Albert Pandiangan, 60, causing a slight injury on his hand, Amar said.

Police were interrogating the 18-year old man, who told them that he was not working alone, Amar said, without providing details.

Indonesia, the world's most populous Muslim nation, has suffered a spate of deadly attacks by militants since the 2002 Bali bombings that killed 202 people.

The Associated Press contributed to this report.

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