South Koreans held a candlelight rally on Monday against a beef import deal with the United States, following clashes that led to dozens of people being detained by police.

A crowd estimated by police at 2,500 gathered in central Seoul to press the government to renegotiate with the U.S. over an agreement critics say does not protect against mad cow disease.

Protesters waved candles, sang songs and called for the agreement to be scrapped. Police said about 7,000 riot police were on duty at the site, a popular gathering place in downtown Seoul.

Thousands of South Koreans, mostly students and other young people, have held similar vigils and street rallies on a near daily basis against the April 18 deal to resume U.S. beef imports.

Police detained 68 demonstrators at street rallies Sunday and in the early hours of Monday that urged the government to scrap the deal. Police say they will seek prosecutions where appropriate.

Several protesters beaten by riot police were taken to hospitals, though there have been no reports of serious injuries.

The protests are perhaps the biggest domestic challenge faced by Lee in his first months in office. Protesters at the rallies regularly call for his impeachment, though so far there does not appear to be broad support for such a drastic move.

Lee last week sought to reassure the country on the safety of U.S. beef, but failed to ease public anger, fanned by media reports questioning the meat's safety.

He has been criticized for making too many concessions on the beef issue to prompt the U.S. Congress to approve a free trade agreement. South Korea and the U.S. agreed to the landmark accord last year to slash tariffs and other trade barriers, but the deal must be endorsed by legislatures in both capitals before it takes effect.

Lee "pursued his interest at the cost of people's sovereignty and lives," Sung Duck-young, a freelance illustrator, said at Monday night's vigil. "It's a betrayal to the people and that's why we are here."

Prime Minister Han Seung-soo called on South Korea's legislature Monday to ratify the free trade deal before its current term ends this week.

The April 18 beef agreement scrapped nearly all the quarantine restrictions imposed by the previous government to guard against mad cow disease.

South Korea suspended U.S. beef imports after the first American case of mad cow disease appeared in December 2003 in a Canadian-born cow in Washington state. A total of three cases of the disease have been discovered in cattle in the United States.

Restricted imports of U.S. beef reached South Korean supermarkets last year, but further shipments were canceled in October after banned parts, such as bones, were found.

U.S. and South Korean officials have repeatedly said that American beef is safe to eat. Beef was on the dinner menu during Lee's April visit to the presidential retreat Camp David for talks with President George W. Bush.

Scientists believe mad cow disease, or bovine spongiform encephalopathy, spreads when farmers feed cattle with recycled meat and bones from infected animals. In humans, Creutzfeldt-Jakob disease, a rare and fatal malady, is linked to eating meat products contaminated with the cattle disease.

The South Korean government was preparing to make an announcement on revised quarantine restrictions and the safety of American beef imports this week, clearing the way to put it back on Korean store shelves.

A government delegation returned home earlier Monday after conducting on-site inspections of beef industry facilities in the U.S., according to the Agriculture Ministry.