SEOUL , South Korea ( CNN ) -- Tens of thousands of South Koreans were demonstrating Saturday on the streets of the capital to protest the government 's decision to import what they say is unsafe U.S. beef . South Korean protesters protest against government 's policy on U.S. beef imports on Saturday . South Korean police estimate that the crowd in Seoul is about 50,000 . No clashes were reported between the protesters and riot police , although ongoing protests have at times turned violent . South Koreans have protested regularly since April when the government announced it would resume importing beef from the United States after a five-year ban . That ban was instituted over a case of mad cow disease in the United States in 2003 . The widespread public protests essentially paralyzed the government of President Lee Myung-bak , who replaced seven top aides this month and plans to reshuffle his Cabinet . Tens of thousands of auto workers in South Korea went on strike Wednesday to oppose the government 's lifting of the ban . After a series of negotiations , Seoul and Washington came up with a revised agreement on June 21 -- one that limits imports to cattle younger than 30 months old . Animals older than 30 months old are considered at a greater risk for mad cow disease , which can be transmitted to humans . The revised agreement also excludes the import of certain parts believed more susceptible to mad cow disease . The initial deal would have allowed the import of all U.S. beef imports . Scientists believe mad cow disease spreads when farmers feed cattle recycled meat and bones from infected animals . The U.S. banned recycled feeds in 1997 . Eating meat products contaminated with the illness has been linked to variant Creutzfeldt-Jakob disease , a rare and fatal malady in humans . Until the 2003 ban , South Korea was the third-largest market for U.S. beef exporters . South Korea 's new pro-U.S. president agreed to lift the import ban in April before a summit with U.S. President George W. Bush . But the move provoked a backlash over health concerns spurred in part by false media reports about risks , along with a sense that South Korea had backed down too easily to American pressure . The government has vowed to get tough with the rallies . In Washington , the White House announced that Bush would visit South Korea on August 5-6 before heading to the Beijing Olympics . Bush had originally been expected to go to Seoul next week when he visits Japan for the G-8 summit , but the trip did not materialize amid the protests . CNN 's Sohn Jie-ae contributed to this report .
Tens of thousands of South Koreans opposed imports of U.S. beef . With imports largely banned since 1972 when mad cow disease found in U.S . Protests essentially paralyzed the government of President Lee Myung - bak .