South Korean anger was triggered by the brutal killing of 33-year-old hostage Kim Sun-Il by his Islamic captors after President Roh Moo-Hyun refused their demand to cancel the deployment of more than 3,000 extra South Korean troops to Iraq starting in August.

Pro- and anti-war demonstrators took to the streets of Seoul and anti-war groups said they would stage mass rallies on Saturday in cities across the country.

"We will stage an all-out struggle by our members to stop the dispatch of troops," said the umbrella Korean Confederation of Trade Unions which groups some 500,000 workers.

Pilots at South Korea's two commercial airlines, Korean Air and Asiana, said in a statement they would boycott flights carrying troops or equipment to Iraq.

South Korea agreed last year to a US request to deploy troops in support of the US-led coalition in Iraq, a decision ratified by parliament in February.

Analysts said the killing of Kim further polarized the country between anti-US opponents of the war and pro-US supporters.

"Opposition to the war and the troop dispatch will grow," said political science professor Lee Chung-Min at Hankook University of Foreign Studies.

But he said there was now no going back for Roh, who would compromise his leadership if he reversed the troop decision.

Several pro-US activists on Thursday torched a portrait of Abu Mussab al-Zarqawi, the Al-Qaeda linked militant who leads the group who beheaded Kim.

Anti-Arab sentiment was also apparent as police stepped up security to protect South Korea's small community of around 30,000 Muslims after some 40 callers threatened to blow up Seoul's main mosque.

The anger of the anti-war activists focused on the government for standing firm on the troop dispatch instead of bowing to the militants' demand, with some directing their rage at the foreign ministry, accusing it of bungling the attempt to free Kim.