SEOUL — South Korean police refused yesterday to allow more candlelight protests against the resumption of American beef imports, just hours after thousands of demonstrators clashed with riot police in the streets of the capital.

The government said it would not tolerate violent, illegal rallies. Authorities used police buses to encircle a plaza in front of Seoul City Hall — the main site for weeks of evening rallies — to prevent protesters from gathering.

Nevertheless, about 1,700 people marched into nearby downtown streets chanting slogans demanding the government of President Lee cancel its decision to lift a ban on American beef.

Thousands of riot police quickly chased them away. There were no immediate reports of serious injuries or clashes.

A spokesman for the protest group, Jang Dae-hyun, said police should cease harsh methods against demonstrators to prevent further violence. “We’ve been supporting peaceful rallies, but the police crackdown is too harsh,” Mr. Jang said.

The hard-line stance came hours after about 15,000 people — some wielding steel pipes and hurling stones at police — demonstrated in the capital, leaving more than 200 protesters and riot police injured.

The rally turned violent after some protesters used ropes to try to drag away police buses used as barricades to prevent them from marching into the presidential Blue House.

Riot police immediately fired water cannons and sprayed fire extinguishers to repel them. Angry protesters attacked police, while police used clubs and shields against the crowd.

Rallies after sunset without police permission are officially illegal. Activists have nonetheless staged daily candlelight rallies to voice fears about the possible health risks of American beef, such as mad cow disease.

The justice minister, Kim Kyung-han, said authorities may have no choice but to use measures such as tear gas to prevent clashes between police and protesters. Tear gas has been banned since 1999.

He also said authorities would arrest those who instigate violent protests, which he said would aggravate national economic difficulties amid rising global oil prices.

As officials began inspecting American beef Friday before it can reach markets, hundreds of labor activists blocked customs storage facilities.

“We are just here to express our opinions. I can’t understand why this government tries to ban our rally,” Kim In-seok, 69, who runs a small construction company in Seoul, said. “Lee Myung-bak will face a serious public backlash.”

American beef has been banned for most of the time since late 2003, when the first case of mad cow disease in the America was discovered.