SEOUL, South Korea — American beef returned to South Korean store shelves yesterday under a new import agreement that has failed to stem anti-government protests, which have raged for weeks and turned central Seoul into a riot zone.

The government said it would take tough action to stop the increasingly violent rallies, which began two months ago with schoolgirls holding candlelight vigils. But the protests have lately also seen club-wielding demonstrators trying to break through barricades of police buses under showers of water cannons.

“What began as a peaceful candlelight rally has changed in nature to a point where it is difficult to see any purity” in its cause, Prime Minister Han told a Cabinet meeting, according to a government Web site.

This week, a prominent Catholic group joined the rallies, with priests and nuns also protesting at a plaza in front of city hall. Some 3,000 people gathered there yesterday evening, police said, and no clashes were immediately reported.

American beef went on sale earlier yesterday without any fanfare and was limited to 440 pounds at a store run by the head of the Korea Meat Import Association.

Large supermarket chains have said they would not sell American beef due to the negative public sentiment, and restaurants across the country have said they would not serve it. South Korean branches of American fast food chain McDonald’s is broadly advertising that they use Australian beef, not American.

Also yesterday, the AP reported that South Korea plans to propose to North Korea that the nations march together at the opening and closing ceremonies of the Beijing Olympics.