A student is in custody after launching a ferocious attack on his teacher and three of his classmates with a cricket bat at an Australian university.
The 18-year-old attacker stood up from his seat armed with the bat and approached the teacher at the front of the statistics class, Australian Federal Police said.
Other students tried to restrain him but he attacked the teacher and three others.
Police called to the Australian National University in the capital Canberra took the attacker into custody, Detective Superintendent Ben Cartwright said.
Mr Cartwright praised the "incredible bravery on behalf of those students to protect the lecturer".
During the assault, one student managed to wrest the bat from the attacker and run away with it, Mr Cartwright said.
The attacker was not known to police or intelligence agencies, and his motivation is unknown.
He has not yet been charged over the attack and police have not released his name.
The four victims were taken to a hospital with serious but non-life threatening injuries, including broken bones, Mr Cartwright said.
Student Max Claessens, 18, whose friend was inside the classroom, said his friend told him that the attacker waited until the class had settled in before he suddenly pulled the bat out of his bag and began hitting people.
The attacker had been a student of the statistics class for four weeks, Mr Claessens said.
"One of the students just randomly out of the blue got up with a bat, struck out at two or three students, before apparently going for the teacher," he said.
"And obviously people were in a bit of shock so they ran out to get help before the guy - who wasn't coming down and was going a bit crazy, to be honest - was restrained."
The student had not previously drawn anyone's suspicions, Mr Claessens said.
"It was just out of the blue. Nothing seemed strange about him," he said.
"I'm a bit surprised to be honest. I didn't expect something like this to happen.
"You can't exactly anticipate someone picking up a cricket bat in a statistics class."
Jolene Laverty was on her way to a lecture when she saw the assailant being led away by police and several of the injured being treated by paramedics.
"There was a man on a stretcher who was being offered morphine, so obviously in a lot of pain with a lot of blood coming from his head," she said.
"And a lot of people in shock as well. A lot of people getting blankets put over them and being looked after by the staff and emergency services."
The attacker was being escorted by two police officers and was talking to them calmly, she said.
The university's deputy vice chancellor, Marni Hughes-Warrington, said she was moved by the courage shown by the students who tried to stop the attack.
"I'm delighted and unsurprised to see such acts of bravery - we know one another, we really care for one another," she said.
"This is an isolated and random incident that's happened in a very caring community. This is a very unusual thing."
The university said counselling services were being provided to anyone affected by the attack.