South Korea has been gripped by labour unrest

At least 12 people, including some policemen, were injured in the clashes.

About 40,000 workers took part in the protest against labour laws under which union leaders can be sued for lost production during strikes.

Three union leaders have committed suicide over the measures that have led to a wave of labour unrest and caused industrial output to drop.

Mourning robes

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

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

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

Organised by the Confederation of Trade Unions (KCTU), the demonstrators appeared prepared for trouble as they assembled outside the city hall, says the BBC's Charles Scanlon in Seoul.

We will stage a stronger struggle if the Roh governmment sticks to its current policy Dan Byung-Ho KCTU head

Masked workers shouting "Revenge for the deaths of our colleagues" brandished steel pipes and threw fire bombs. Riot police charged and fought back with clubs in what became running battles through the streets of central Seoul.

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 says.

The former human rights lawyer 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, he says.

The union says it plans to stage a general strike on Wednesday.

"We will stage a stronger struggle if the Roh governmment sticks to its current policy," KCTU head Dan Byung-Ho warned in a speech.