The incident occurred around 7pm Tuesday when the 17-year-old was playing basketball with his friends at the court in Lai Kok Estate, Sham Shui Po. The teen had physical contact in a quarrel with the four, who claimed to be members of a triad society, but the two sides departed soon.

Later, when the teen entered a shop in Lai Kok Shopping Center, the four, including one wielding a 30cm-long knife, ambushed him and threw punches at him. The outnumbered teen sustained injuries to his left eye, left knee, and mouth.

The four were long gone when paramedics arrived and sent the teen to Caritas Medical Centre.

The case was handled by the police's Sham Shui Po anti-triad unit. The four, aged 15 to 18, were soon arrested in Sham Shui Po district on Thursday and Friday for claiming to be an office-bearer of a triad society, assault occasioning actual bodily harm, and possessing an offensive weapon.

Among the four, the 15-year-old who once wielded the knife and another 18-year-old were remanded in custody. The remaining two, aged 15 and 17, were released on bail and must report to the police in late November.