By William Sim

June 29 (Bloomberg) -- South Korean authorities detained 55 people after overnight clashes between protesters and police left more than 100 injured on each side as demonstrations against U.S. beef imports continued.

About 15,000 demonstrators paraded along the 12-lane Sejong Avenue last night, marching toward the office of President Lee Myung Bak and calling on him to resign. Police used water cannons and shields to drive them back.

As many as 112 protesters were treated for injuries at local hospitals, according to the Web site of AntiMadCow, an umbrella group of South Korean consumer groups, food safety advocates and student activists, which is organizing the protests.

Fourteen police officers were seriously injured and more than 100 others required medical attention, according to a spokesman at the Seoul Metropolitan Police Agency, who declined to give his name.

Lee has been trying to contain a backlash from his agreement in April to resume imports. South Korea stopped accepting American beef shipments in 2003, when the U.S. discovered its first mad cow case. Some cuts were imported in 2006 and the ban was reinstated in October. Lee was forced to publicly apologize for his handling of the negotiations and fired many of his aides.

The U.S. and South Korea agreed June 21 that South Korea will import beef only from animals younger than 30 months, which are thought to be at lower risk for mad-cow disease. The U.S. agreed to verify the age of its exports.

Tensions Rise

Tensions escalated after the government on June 26 posted a notice in its official gazette that South Korea would resume importing U.S. beef, the final bureaucratic hurdle before shipments begin.

Mad cow disease is a brain-wasting livestock illness that scientists say is spread in cattle by tainted animal feed. Eating contaminated meat from infected animals can cause a fatal human variant that has been blamed for the deaths of 151 people in the U.K., where it was first reported in the 1980s. Beef from cows younger than 30 months is thought to be at lower risk.

U.S. Secretary of State Condoleezza Rice sought to reassure South Koreans that American beef is safe to eat during her visit to Seoul yesterday.

South Korea, prior to the ban, was the third-largest importer of U.S. beef. Lee, meanwhile, is working to secure a free-trade agreement with the U.S., a deal that's meeting resistance in the U.S. Congress.

To contact the reporter on this story: William Sim in Seoul at wsim2@bloomberg.net.