Tens of thousands of South Koreans rallied against a government decision to import U.S. beef in the largest demonstration in a month of almost daily protests.

A crowd estimated by police at 38,000 people filled a plaza in front of city hall on Saturday night. Protesters lit candles, waved placards and chanted slogans criticizing President Lee Myung-bak.

Lee's office declined to comment on the rally.

South Korea agreed in April to reopen what was formerly the third-largest overseas market for U.S. beef. It had been shut for most of the past 4 1/2 years following the first U.S. case of mad cow disease in a Canadian-born cow in Washington state in 2003.

That deal, coupled with some sensational media reports, sparked fears of mad cow disease and triggered protests calling for scrapping or renegotiating the agreement.

Before the 2003 ban, South Korea was Nebraska's second-largest beef market, valued at $108 million annually. Japan was the biggest overseas market for Nebraska beef exports.