Almost 50,000 people protested in Seoul today against a US beef import deal and the policies of the new president, whose government has faced weeks of street rallies.
Polls show that most South Koreans oppose a deal President Lee Myung-bak struck in April to open the market to US beef.
The protests began in early May by people concerned about possible mad cow disease in US beef but later grew into a forum where a wide variety of people gathered to air grievances against Lee, who won a December election by a landslide.
Lee has seen his support plummet and analysts say he cannot implement reforms such as privatising state-run firms and revamping pension systems unless he wins back the public.
Rallies last weekend left hundreds of protesters and conscripted riot policemen wounded, prompting religious leaders to join the rallies in the hope of calming things down.
South Korean and US trade envoys reworked the beef deal in June with a private-sector agreement that limits trade in beef to cattle under 30 months of age, thought to pose a low risk for mad cow disease, and prohibits shipments of risky body parts.
US and South Korean leaders have said there is no scientific evidence that shows the US beef headed to South Korea poses a risk for the brain-wasting disease.
US beef returned to shelves in South Korea this week, once the third-largest overseas market for the product with annual sales of about $850 million before Seoul banned sales in 2003 due to an outbreak of mad cow disease in the US.
Major retailers refused to sell the product out of fear of antagonising Koreans, but the few independent butchers who offered US beef quickly sold out. People lined up to buy the product that sells for at least half the price of Korean beef.