Thousands of demonstrators are gathering in Seoul for the latest protest against the resumption of US beef imports into South Korea.
A heavy security operation is under way in the capital after organisers promised it would be the biggest yet.
The protest comes after South Korea's entire cabinet offered to resign amid the continuing protests.
Yonhap news agency said President Lee Myung-bak was likely to accept the resignations of several ministers.
The protesters are angry that South Korea has agreed to resume imports of US beef, which were suspended in 2003 after an outbreak of BSE, or mad cow disease, in cattle there.
They accuse the government of failing to safeguard their health, despite assurances that the beef is safe.
Some 20,000 riot police have been mobilised in Seoul, and thousands more elsewhere in South Korea, ahead of Tuesday night's protests, which organisers say could attract up to one million people.