Police have identified a group of youths who were caught on a video beating up another schoolboy in an HDB stairwell, and are investigating the matter.

One of the boys in the video, which has gone viral and shocked many with its level of violence, was wearing a white singlet from St Andrew's Secondary School.

The school said that some of its students were involved in the incident, which took place last Saturday.

Its principal Lucy Toh said: "We take all incidents of fighting seriously, and will continue to educate our students on anger management and appropriate behaviour."

In the 25-second video - which has been viewed more than a million times on Facebook under the title "Say No To Bullies!!", shared more than 35,000 times and drawn more than 4,400 comments - two youths can be seen raining blows on a smaller boy. The victim did not fight back, while a group of other boys simply watched.

When he fell to the ground, one of the attackers, who was holding what appears to be a floorball stick, leapt up and kicked him.

The Ministry of Education said it was aware of the incident, but could not comment further as the case is under police investigation.

Counsellors said the attack could be a form of gang discipline.

Mr Wilson Tan, executive director of Youth Guidance Outreach Services, said: "These videos could have been uploaded on social media to threaten other youths, or as a way of getting attention."

He added: "The two boys definitely need counselling and intervention, and support is needed for the boy who was beaten up."

Many netizens expressed shock and outrage. Madam Cindy Hiang, a 42-year-old administrative executive who has two sons, said: "It's scary that the boys can be so violent at such a young age."

A 34-year-old civil servant, who wanted to be known only as Mr Veera, said: "These boys need to be corrected before they get out of hand."

He had sent the video to citizen journalism website Stomp on Thursday, after receiving it via a WhatsApp group chat with friends.

A spokesman for the police said they "take a stern view of lawless and violent public behaviour, which could threaten the sense of safety and security in the community".

"Any person who resorts to violence and riotous behaviour will be dealt with severely, in accordance with the law."