By Jack Kim

SEOUL (Reuters) - The South Korean government vowed on Sunday to crack down on any more violent protests, a day after dozens were arrested during a rally against labor reforms, the largest street protest of President Park Geun-hye's term.

Organizers say they will take to the streets again on Dec. 5.

More than 60,000 people took part in Saturday's protest, according to police, and a group of a few dozen fought with the police at the front line, trying to break through barricades of police buses blocking off downtown Seoul's main thoroughfare.

Police used water canons to disperse the crowd and sprayed liquid laced with an irritant found in chilli pepper to fight off protesters swinging metal pipes and sharpened bamboo sticks.

"The government was fully prepared to guarantee a lawful and peaceful rally, but some people came prepared with illegal equipment such as steel pipes and conducted a violent protest," Justice Minister Kim Hyun-woong told a news conference.

"These activities were a grave challenge to law and order and public authority, and they will not be tolerated."

The police arrested 51 people and are questioning them on various charges including illegal protest, assaulting police officers and destroying public equipment.

The police said about 10 protesters were injured, including a member of a militant farm activist group who was knocked down by a water canon blast. He was in stable condition after emergency surgery on Sunday, a police official said.

Some of the country's most militant labor and activist groups were involved in the protests, including Han Sang-gyun, the president of the Korean Confederation of Trade Unions, who is wanted under a warrant for organizing previous illegal rallies.

"It was led by some of the most organized elements -- labor, farm, anti-poverty activists, which was a little different from when there was more public participation," said Yu Chang-seon, an independent political commentator.

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Protestors say the labor reforms benefit only the country’s huge family-controlled conglomerates, and make it easier to fire workers.

Park, who had left earlier on Saturday for Turkey to take part in the summit of G20 nations, has seen her public support ratings fall recently over a decision to replace privately published school history textbooks with a government version.

The protests do not, however, appear to pose an immediate threat to Park or her conservative Saenuri Party, which is well ahead in opinion polls, scoring 39 percent in a Gallup survey of 1,012 people released on Friday, while the largest opposition party, New Politics Alliance for Democracy, polled 22 percent.

Parliamentary elections take place next April.

(Additional reporting by Hooyeon Kim; Editing by Will Waterman)