Station Inspector Sofian Othman was on leave and had just bought himself a sandwich and a bottle of water in Bedok Mall for dinner at about 5.45pm on Dec 27 when he heard a commotion behind him.

A disoriented 18-year-old youth, dressed in a singlet and jeans, and brandishing a 10cm-long pair of scissors, was shouting and gesturing aggressively. He was also going into shops and sweeping products off shelves.

Observing him for about a minute, Station Insp Sofian, 35, waited till the teenager was distracted. Putting down his takeaway meal, the cop, with 14 years of experience, disarmed the teen from behind at watch shop H2 Hub. He then pinned the teen down with the help of four other men.

Station Insp Sofian said he was heartened by the reaction of the public in view of the current security climate and noted the efficacy of the SGSecure programme.

He said: "I had coordinated with one of the men. I was glad that the members of the public were alert and came forward to assist. I'm happy that we can rely on the public in a crisis.

"It was important to take him down as the mall was crowded that evening."

Station Insp Sofian offered the teenager his bottle of water to calm him down.

The teenager, with a history of mental health problems, was arrested for causing public nuisance after police arrived on the scene.

Station Insp Sofian, who was speaking at a media interview on yesterday, has intervened in other situations while off duty, but none were as dramatic as the incident on Dec 27.

He said: "The other incidents were mostly arguments.

"I'm alert 24 hours, even when I'm overseas. It is a policeman's first instinct. I didn't want him to cause harm to others and damage property."

Investigations are ongoing.

The maximum penalty for causing public nuisance is a $1,000 fine.