South Korean police said they were questioning scores of protesters over violence that broke out at a rally denouncing the government's handling of a ferry disaster that killed more than 300 people a year ago.
About 80 people were being questioned on Sunday over the demonstration, the official said. Twenty others, who were also detained, have been released.
Police clashed with thousands of protesters, blocking their way to the presidential palace, where they hoped to demand a more vigorous government response to the ferry disaster.
About 13,000 police and 470 police buses were deployed in the area around Seoul's main ceremonial thoroughfare, an official of the Seoul Metropolitan Police Agency said.
Saturday's rally, organised by a group representing families of the victims, was the largest in recent weeks as the country marked the first anniversary of the April 16 disaster.
Police said they used buses to barricade marchers' route to the presidential Blue House, and deployed water cannon and pepper spray. Several police buses were damaged.
A fire department official said nine protesters and three police were taken to hospital while three protesters received first aid at the site.
The rally began at midday, with a crowd demanding the government allow an independent inquiry into the disaster and make an immediate decision to raise the Sewol ferry.
The ferry was on a routine journey from Incheon to the southern island of Jeju when it capsized making a sharp turn and sank. Among the 304 victims were 250 students on a class trip.
It was later found to have been structurally unsound and overloaded with cargo.
Anger still runs high among the families of victims. They say the government let them down again this week, by failing to announce a plan to raise the ferry by the first anniversary of the event.
President Park Geun-hye said on Thursday the government would begin preparing to raise the 6,800-tonne ship, her clearest indication yet of a retrieval plan, before she left on an 11-day trip to Latin America.
The country's umbrella labour federation joined the rally and vowed to join forces with victims' families to continue protests, calling for more rallies next weekend, a federation spokesman at the protest said.