STORY: The incident took place at about 10:20 p.m. (1320 GMT). A large number of people fell down in a narrow alley during the Halloween events, Choi Sung-beom, head of the Yongsan Fire Station, said.
The number of casualties could rise as the rescue effort was still under way.
President Yon Suk-yeol presided over an emergency meeting with senior aides and ordered emergency medical teams to the area, his office said.
Authorities said they were investigating the exact cause of the incident.