Fire officials said two people had been confirmed dead and 12 others were unresponsive after the accident in Seongnam, just south of Seoul. They said 10 others were seriously injured.
The officials said the victims were standing on the ventilation grate while watching an outdoor performance by girls’ band 4Minute, which is popular across Asia. About 700 people had gathered to watch the concert, which was part of a local festival, the Yonhap news agency reported.
The officials spoke on condition of anonymity, citing department rules.
The cause of the accident wasn’t immediately known.
The accident came as South Korea is still struggling to deal with a ferry disaster in April that left more than 300 people dead or missing.