Fire officials, speaking on condition of anonymity because of office rules, earlier said that the victims were standing on the 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 YTN television network, citing unidentified witnesses, said many of the spectators were female students. The victims weren't immediately identified.
A video from the scene recorded by someone at the concert that later ran on YTN showed the band continuing to dance for a while in front of a crowd that appeared to be unaware of the accident.
Meanwhile, dozens of people were shown standing next to the ventilation grate, gazing into the dark gaping hole where people had been standing to watch the performance. YTN said the ventilation grate was about 3 to 4 meters wide. Photos apparently taken at the scene showed that the ventilation grate reached to the shoulders of many passers-by.
The cause of the accident wasn't immediately known.
The collapse came as South Korea still struggles with the aftermath of a ferry disaster in April that left more than 300 people dead or missing.
For a time, the sinking jolted South Korea into thinking about safety issues that had been almost universally overlooked as the country rose from poverty and war to an Asian power.
The tragedy exposed regulatory failures that appear to have allowed the ferry Sewol to set off with far more cargo than it could safely carry. Family members say miscommunications and delays during rescue efforts doomed their loved ones.
Analysts say many safety problems in the country stem from little regulation, light punishment for violators and wide ignorance about safety in general - and a tendency to value economic advancement over all else.