Around 100 people were attending the service in the town of Sleman in Yogyakarta province on Java island when a man barged in wielding a one-metre-long sword and began attacking terror-stricken people, seemingly indiscriminately.
![The suspect pleaded guilty to threatening and offensive behaviour [Task & Purpose]](data:image/svg+xml;charset=utf8,%3Csvg%20xmlns%3D'http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg'%20width%3D'100'%20height%3D'100'%20data-ring-placeholder%3D'1'%3E%3C%2Fsvg%3E "The suspect pleaded guilty to threatening and offensive behaviour [Task & Purpose]")
"Four people have been injured in the incident -- quite seriously -- but we still cannot determine the perpetrator's motive," Yogyakarta police spokesman Yulianto told AFP.
Extremists in the world's most populous Muslim-majority country have mounted a series of attacks against Christians and other minorities.
A few minutes after the service started, a congregation member ran into the church with a bleeding head chased by a young man holding a sharp weapon, said worshipper Andhi Cahyo.
"Everybody started panicking and screaming. I was scrambling to save my wife and children," Cahyo told AFP.
People fled through another door as the attacker ran amok inside the church.
He destroyed some books and a Virgin Mary statue with his sword, said Cahyo, and attacked 81-year-German priest Edmund Prier who was standing at the altar.
Prier, who has been living in Indonesia for decades, is now an Indonesian citizen.
Police arrived soon after the attack and fired a warning shot but the attacker refused to surrender.
"After the warning shot was fired, the attacker charged towards the officer with his sword. The officer then shot him below his stomach, but he managed to injure the cop before being subdued," Cahyo said.
All victims have been taken to hospital for treatment. Police said the man was a university student in his early 20s but could not confirm if the incident was related to terrorism.
"For now we cannot conclude this is related to terrorism. We need to dig out more details and question the perpetrator," said spokesman Yulianto.
The man was currently being treated at Bhayangkara hospital and could not questioned, Yulianto said.
Indonesia is home to significant numbers of Christians, Hindus and Buddhists.
In 2016 several children were injured after a man threw Molotov cocktails at a church during a Sunday service.
On Christmas Eve 2000 the Al-Qaeda-linked group staged coordinated bombings of churches in and eight other cities which killed 18 people and injured many more.
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