President Roh Moo-hyun came to office with support from the unions

"It's not right for workers to resort to illegal and violent means," Mr Roh's staff quoted him as saying.

The violence happened during protests by supporters of the Korean Confederation of Trade Unions (KCTU), on Sunday.

The organisation has called a strike for Wednesday.

The KCTU is calling on the President, a former labour lawyer, to repeal laws that allow union leaders to be sued for lost production during strikes.

About 35,000 workers took part in the protests which turned violent when the demonstrators hurled fire bombs at riot police.

We'll step up our actions on 12 November with a more powerful strike KCTU statement

The police said at least 44 officers were wounded, while union leaders said 100 workers were hurt, 56 seriously.

The workers say they are out to avenge three colleagues who killed themselves in recent weeks in protest against government policies.

Some, wearing mourning dress, marched with pictures of the dead union leaders.

Others waved red and white flags and banners, including one that read "No more killing".

Several thousand riot police were mobilised to seal off government offices and the American embassy.

The KCTU has become frustrated with the policies of President Roh Moo-Hyun, our correspondent in Seoul, Charles Scanlon, said.

Mr Roh came to office earlier this year with support from the union movement, but he has since taken a harder line, following a series of damaging strikes at some of South Korea's leading companies.

"This isn't the end of our protests. We'll step up our actions on 12 November with a more powerful strike," the KCTU said in a statement.