The shocking event came from nowhere.
One moment, students were relaxing easily on the lawn on one of the first truly hot days of the spring, the next moment there was mayhem with people shouting: "Just get down. Just get down".
The police were on the scene very quickly and identified a man in black wielding the knife. According to witnesses, he made a run for it and officers brought him down.
Betty Hannaford was on the edge of the oval as the arrest was made.
"Police turned up in numbers," she said.
"We saw him being tackled on the ground. they chased him across the lawn."
Mrs Hannaford said there was no panic. "We just tried to keep out of the way".
She was a member of the public visiting libraries on the campus at the time. Her group was heading from the Menzies Library towards the Chifley Library when the event erupted in front of her and her party.
"Just as we got to the lawn, there was a great tumult, and somebody shouted, 'Somebody's been stabbed. Don't go there'."
The man who she said had a black top on was led away by police after he had been brought down.
Within minutes of the arrest, the lawn was cordoned off by rows of police and blue tape. One young male witness was treated by paramedics for some sort of blow to the head. He was taken to a police car still clutching the ice-pack to the side of his skull. Police said he was a witness about to give a statement.
And then, in the surreal way of these things, an apparent normality quickly returned over the campus apart from the immediate area of the alleged stabbing. Life went on in the bookshop and libraries.
Inside the Chifley Library, books were being sought. Students sat and studied as though it was just another normal day, even as, a matter of metres away, police and security officers from the university kept guard over the scene of the alleged crime on the outside of the building.
Small groups of students gathered to share what information they had - which, it turned out, was not much. Rumours spread. The atmosphere was sombre and bemused. The campus of one of Australia's most prestigious universities is not the kind of place where these things are meant to happen.
But behind the apparent normality, social media started buzzing, with pictures of the arrest circulating.
Students surmised on the motive of the alleged attacker. The general assumption was that he was a student - but that was pure speculation on their part.
The area in front of the library remained closed until well into the evening as police continued to investigate the event. The ANU urged students to keep away from the Fellows Oval and the Chifley Meadow while police continued their investigation.
The ANU emailed all students and staff saying: "We encourage anyone who has been distressed by this to make use of the following resources.
"Staff can access support and counselling through the Employee Assistance program on 1800 808 374.
"Students can access support via the ANU Crisis Support Line on 1300 050 327 or via SMS Text message service on 0488 884 170."
The university authorities urged people who might know anything to go to City Police Station to share the information.
