Dozens were injured in the largest protest yet over an agreement to resume U.S. beef imports to South Korea, according to authorities and media reports Saturday. A crowd estimated by police at 65,000 demonstrated in central Seoul on Friday night. Some marched on a road leading to the presidential Blue House but were blocked by a barricade of police buses. Riot police later clashed with demonstrators who tried to march to the presidential office through a back alley.

About 25 riot police were hospitalized and protesters smashed the windows of four of the buses, said an officer at the Seoul Metropolitan Police Agency. The officer declined to give his name, citing office policy.

The injuries were not critical, he said.

The officer had no information on the number of protesters injured and rally organizers were not immediately available for comment. News cable channel YTN said "dozens" of protesters were hurt.

South Koreans have been taking to the streets for weeks to criticize President Lee Myung-bak for his handling of an April agreement with Washington to restart imports of U.S. beef.

On Thursday night, a crowd estimated at 25,000 people staged a candlelight protest downtown. Organizers have pledged that rallies will continue nonstop though Sunday.

Many South Koreans fear that the beef deal fails to protect the nation from mad cow disease (news) by allowing beef from older U.S. cattle, considered at greater risk of the illness.

Protesters complain Lee has ignored their concerns, behaved arrogantly and given in to U.S. demands.

A contrite Lee vowed Friday to take a "humble attitude."

"I will open my ears in a more humble attitude and listen to the people," Lee said in a speech Friday to commemorate Memorial Day, a national holiday. He did not directly mention the beef dispute.

Later Friday, presidential spokesman Lee Dong-kwan said all eight senior presidential secretaries - including himself - had offered to resign.

In South Korea, senior officials sometimes offer to step down during times of crisis to deflect or diminish criticism of an embattled leader.

The spokesman said Saturday it was not clear whether or when the president would accept the resignations.

South Korea's government said last week it would begin allowing imports this week, but withdrew the plan at the last minute Monday, apparently fearful of a public backlash.

The government also said it has asked the United States to refrain from exporting beef from cattle 30 months of age or older. Still, it stopped short of directly asking Washington for a renegotiation of the deal and failed to calm public anger.

On Friday, President Lee ruled out any formal renegotiation, saying it might spark a trade dispute that could affect the country's export-driven economy, especially the key auto and semiconductor industries.

"We are a trade-dependent nation," Lee said during a meeting with Buddhist leaders. "Demanding renegotiation will cause an enormous problem due to trade friction."

Lee said he would seek other ways to keep beef from older cattle from entering the country, and that the United States is "actively cooperating" with Seoul to find a solution.

U.S. beef has been banned from South Korea for most of the past 4 1/2 years since the first case of mad cow disease in the U.S. was discovered in late 2003. Two subsequent cases were found.

Scientists believe the disease spreads when farmers feed cattle recycled meat and bones from infected animals. The U.S. banned recycled feeds in 1997.

In humans, eating meat products contaminated with the illness is linked to variant Creutzfeldt-Jakob disease, a rare and fatal malady blamed for the deaths of over 150 people worldwide, mostly in Britain.