Seoul police have apprehended three more people suspected of threatening to commit copycat crimes in Seoul following a recent stabbing rampage on the outskirts of the capital, officials said Saturday.

They were detained Friday as police vowed to step up vigilance after 14 people were wounded Thursday when a man rammed a car onto a sidewalk and attacked people with a knife at a department store in Seongnam, south of Seoul.

The incident followed another stabbing spree at Seoul's Sillim Station on July 21, which left one dead and three others wounded.

The three suspects include a 20-something man who wrote an online message threatening to "kill all" at Wangsimni Station in Seoul, the Seoul Metropolitan Police Agency said.

Police also held an underage person who threatened to slay five people at the entrance of a school in the capital and a man in his 20s who wrote he would brandish a knife at Seoul's Hanti Station.

On Thursday, the police formed a team to deal with such online threats and warned of stern punishment.

Last month, Seoul police nabbed two people who also threatened to commit homicide following the incident at Sillim Station. (Yonhap)