SEOUL (AFP) - South Korean President Park Geun Hye on Tuesday (Nov 24) called for a ban on masks at demonstrations, less than two weeks after huge anti-government protests rocked Seoul, as she warned "terrorist elements" may infiltrate demonstrations.

The president also drew parallels between masked protesters and the Islamic State in Iraq and Syria (ISIS) - prompting organisers of the recent rally to say her comments had left them "speechless".

More than 60,000 people protested against the conservative government's push for labour reform and state-issued history textbooks in Seoul on Nov 14, in the biggest protest in the country for nearly a decade.

Many scuffled with police, who responded with water cannon and liquid pepper spray, leaving dozens injured and one protester in a critical condition.

Police have come under fire for what critics describe as excessive use of force, while about 200 demonstrators are being investigated after dozens of police buses were damaged.

Park described the clashes on Nov 14 as an "unacceptable" incident and called for "strong measures" against the protesters, especially those in masks.

"At a time when acts of terrorism are taking many lives around the world, some terrorist elements may sneak into such protests and pose a threat to the lives of our people," she said at a cabinet meeting on Tuesday.

"In particular, masks in protests should not be tolerated. Isn't that what the ISIS is doing these days, with their faces hidden like that?" she added.

However, the Korean Confederation of Trade Unions - a militant umbrella labour union and a leading organiser of the Nov 14 rally - accused the president of trying to stifle dissent.

"This is really pathetic," KCTU said in a statement.

"We never expected her to have any insight in democracy but now we are left to wonder if she has any sense of judgment."

KCTU added: "Her statements over ISIS and masks ... left us simply speechless."

The Nov 14 demonstration saw police surround demonstrators with barricades composed of hundreds of police buses, with protesters - some wearing masks - tying ropes to the vehicles to try to move them.

Police faced criticism for aiming water cannons directly at protesters, including one who suffered concussion and remains unconscious and in a critical condition.

The protest focused on a series of issues, including what Seoul's planned labour reform critics said would make it easier for firms to fire workers, and the controversial imposition of state-issued history textbooks in schools.

The textbooks have become a bitter ideological battleground between left and rights.

The organisers of the Nov 14 rally have vowed to launch another massive protest on Dec 5.