Tens of thousands of people, holding lighted candles, rallied in Seoul and other cities on Saturday night against a beef import deal their government signed with Washington, claiming that it fails to protect Koreans from the danger of mad cow disease.

The Korean government inked a controversial agreement with the United States on April 18 to open its market to most American beef cuts. Many angry Koreans have since taken to the streets for protests almost daily.

Public anger against the beef deal escalated after the government officially announced Friday that it will enforce the deal within days. The decision was announced, hours before President Lee Myung-bak was to return home from an official four-day trip to China.

"Go away, mad cow!," shouted a protest leader on a podium in front of the Seoul City Hall. The crowd, estimated at 30,000, picked up the chant, pumping their fists into the air.

Hundreds of people in the crowd began marching toward the presidential office of Blue House, raising concerns that they might clash with riot police deployed in the area.

They demanded renegotiation of the beef deal but the government has already said no.

Opposition party lawmakers were also stepping up their offensive against the Lee administration, demanding that the Cabinet resign en masse to take responsibility for making a faulted beef deal with the U.S.

"The President was picked by people. Could he resist calls from people?" said Kang Ki-gap, floor leader of the minor opposition Democratic Labor Party, as he joined the candlelight rally.

Organizers predicted that Saturday's rally would draw as many as 100,000 protesters, the largest in memories in recent years. Earlier in the day, 5,000 people marched down the streets as they headed to the City Hall for the rally.

Authorities deployed 12,000 police in a show of force against violence.

Similar protests were held in other cities, including Busan, Gwangju and Daejeon, but there were no reports of violence or clashes with police.

Protesters put up large signs which read: "No government can win over people" or "Down With Lee Myung-bak."

Public anxiety over the beef deal was fueled by some media reports that Koreans are more genetically susceptible to the deadly animal disease.

Though the beef issue wasn't a part of a free trade deal signed by Korea and the U.S. in June last year, Washington's lawmakers have said they won't approve the deal unless Seoul fully reopens its market to American meat.

President Lee, a former star executive of Hyundai construction company who took office three months ago after a landslide victory in the December presidential election, has seen his approval rating plummet to some 20 percent.

Korea, once the world's third-largest buyer of U.S. beef, had banned imports of the American meat in 2003 after a case of mad cow disease was found in the United States.

Scientists believe mad cow disease is spread when farmers feed cattle with recycled meat and bones from infected animals. It is thought to cause the fatal human variant mad cow disease, called Creutzfeldt-Jakob disease.