Many thousands of demonstrators are rallying late into the night in Seoul in the latest protest sparked by anger over US beef imports into South Korea.
Protesters, monitored by thousands of riot police, demanded the resignation of President Lee Myung-bak.
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.
Anniversary
Some 20,000 riot police were mobilised in Seoul, and thousands more elsewhere in South Korea, ahead of Tuesday night's large protests.
"This is clearly a struggle for democracy - who will make the decisions for the welfare of the people?"
Fears were raised about the potential for confrontations between protesters and police, or protesters and some conservative groups who have pledged to hold counter-demonstrations - but there have been no reports of violence so far.
The protests come on the 21st anniversary of demonstrations in 1987, seen as pivotal in South Koreans' struggle for democracy.
Regular candlelit vigils have been held for more than a month since the government announced it would resume beef imports. At some there have been clashes between police and protesters, with scores of demonstrators arrested.
Earlier on Tuesday, police stationed huge shipping containers at the Kwangwhamun intersection in central Seoul, trying to block protesters' access to government buildings and the presidential Blue House.
Reports said some containers had been greased by police in an effort to prevent protesters climbing over them.
"The atmosphere is great," Christian Jun Park, a protester, told the BBC News website. He said demonstrators had occupied a large swathe of central Seoul.
He said the protesters ranged from schoolchildren to activists to public-sector workers, and that the protests were not solely concerned with beef imports, but also covered a number of other issues including the government's plans to privatise sections of industry.
"This is clearly a struggle for democracy - who will make the decisions for the welfare of the people?" asked Mr Park.
Delegation to US
The cabinet move came in a regular scheduled meeting.
Political price paid in beef row
"The prime minister and the entire cabinet members offered to resign, taking responsibility for the recent developments," a spokesman for the prime minister told journalists.
A presidential spokesman had no immediate comment on whether the resignations would be accepted.
Eight top presidential secretaries have already volunteered to step down over the row.
Mr Lee's administration has been severely shaken by the dispute, which comes only three months after he took office.
Last week, in its first electoral test since last year's general election, his ruling party suffered a heavy defeat in local polls.
In a bid to ease concern over the beef deal, the government has now asked the US not to export beef from cattle over the age of 30 months.
A delegation are currently in the US to seek assurances on this issue.
But the government says it will not seek to renegotiate the beef agreement in its entirety, saying this would threaten a free-trade pact with the US which has been linked to reopening beef imports.
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