Thousands of protesters hurled firebombs and stones Saturday at riot police who blocked a demonstration against government spying on civilians.

The police fired tear gas to drive back 20,000 students and dissidents who tried to march downtown after an anti-government rally attended by more than 100,000 opposition supporters.The health of opposition leader Kim Dae-jung, meanwhile, deteriorated as his hunger strike against the government of President Roh Tae-woo entered its sixth day. Aides said he might have to be hospitalized.

"Down with Roh Tae-woo!" yelled students hurling rocks and firebombs at police.

There were no immediate reports of arrests or injuries.

In southern Seoul, 15,000 students staged anti-government marches. Students burned a U.S. flag at a street rally and dispersed peacefully while police stayed at a distance, apparently to avert violence.

Protesters were demanding the dissolution of the Defense Security Command military intelligence, which has conducted illegal and widespread espionage on more than 1,300 civilians.

In an apparent effort to divert attention from the domestic political turmoil, Roh on Saturday said he was launching a war on escalating crime and promised to take extraordinary measures to crack down on criminals.

At the rally, thousands of people waving white Korean flags jabbed their fists in the air while shouting anti-Roh slogans.