Photos of the scene in Seongnam, just south of Seoul, showed a deep concrete shaft under the broken grate. Seongnam city spokesman Kim Nam-jun announced the deaths in a televised briefing and said 11 other people were seriously injured.
A man who was involved in planning the concert was found dead early Saturday in an apparent suicide.
Fire officials, speaking on condition of anonymity because of office rules, said 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. Fire officials said many of the dead and injured appeared to be commuters who stopped to watch the concert after leaving work. Most of the dead were men in their 30s and 40s, while five were women in their 20s and 30s, they said.
Kim said it was believed that the grate collapsed under the weight of the people. Prime Minister Chung Hong-won visited an emergency center in Seongnam and urged officials to focus on helping the victims' families and ensure the injured get proper treatment, Kim said.
A video recorded by someone at the concert that was shown on the YTN television network showed the band continuing to dance for a while in front of a crowd that appeared to be unaware of the accident.
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 (10 to 12 feet) wide. Photos apparently taken at the scene showed that the ventilation grate reached to the shoulders of many passers-by.
On Saturday, Kim said an employee of Gyeonggi Institute of Science and Technology Promotion was found dead at around 7 a.m. in Seongnam. The man had been questioned by police on Friday over the accident. Police inspector Park Jeong-ju said the man was believed to have leaped from the top of a 10-floor building.
Gyeonggi Institute of Science and Technology Promotion was one of the sponsors of the concert, which was organized by business news site Edaily.
The collapse came as South Korea is still struggling 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.