The police are looking into an incident where a concertgoer is heard shouting a gang slogan during the concert of South Korean girl group Twice on Sunday. Similar action in the past, where gang slogans were chanted in public, have prompted police investigations.
A seven-second video clip of the incident on Sunday has gone viral after it was first posted on Twitter.
In the video, a voice could be heard chanting "sah lak kau", or "369" in Hokkien, as the nine-member K-pop group sang and danced on stage. The slogan is synonymous with the gang Sio Kun Tong.
The concertgoer's face was not shown in the video.
In response to queries, the police said they are aware of the incident and are looking into it.
Under the Societies Act, it is illegal to join or act as a member of an unlawful society.
It is also illegal to attend a meeting of an unlawful society.
The Twitter user who first uploaded the video claimed the voice in the clip belonged to his brother and that he had shouted the gang slogans during the concert as he did not know the words to a chant the fans were shouting out.
Last November, four men were charged for their alleged involvement in unlawful societies after videos of them shouting gang slogans at funeral processions were circulated online.
Anyone convicted of being a member of an unlawful society may face a fine not exceeding $5,000, a jail term of up to three years, or both.
The concert in Singapore was Twice's second, having performed here last year as well. About 8,500 fans turned up for the three-hour Twiceland Zone 2: Fantasy Park, at the Singapore Indoor Stadium.