SEOUL, NOV. 28 -- A youth hurled a tear gas grenade at government presidential candidate Roh Tae Woo today, and thousands of riot police broke up demonstrations in Seoul and other cities in a show of strength.

The grenade missed Roh, at a rally in the port of Inchon outside Seoul, and failed to go off. Officials said it glanced off a campaign aide. Scores of police seized the attacker and dragged him away. Protesters shouted, "Execute Roh!"

Thousands of police were guarding Roh after reports that radical students would stage an attack. He has escaped a series of assaults with firebombs, rocks and tear gas at campaign rallies leading up to the Dec. 16 presidential elections.

Kim Young Sam, regarded as the main opposition candidate, told a rally in Taegu that the government was using bribery and corruption to try to win the elections. Korean news reports estimated the turnout for Kim at 600,000 people while opposition officials put the figure at 1 million.

About 20,000 riot police were deployed across the country after the National Coalition for Democracy called for protests in 26 cities to demand that Roh quit the race.

Violence erupted in Kwangju, Chonju and Chongju. Police used tear gas, and protesters attacked offices of the governing Democratic Justice Party.

Still, the rallies attracted relatively small audiences and there were no protests in many of the designated cities. The few hundred people who showed up for the protest in central Seoul were ringed by 5,000 riot police.

In Kwangju, police using tear gas dispersed about 200 students who hurled firebombs and rocks in an anti-Roh protest.

A rally by opposition candidate Kim Jong Pil in Pusan was disrupted briefly when a young man leapt onto the dais, cut his finger and wrote a slogan in blood denouncing the Korea Broadcasting System. Opposition supporters contend the state-owned network slants its coverage to favor Roh.

"Roh Tae Woo, Roh Tae Woo!," about 40,000 spectators chanted at Inchon as Roh attacked the opposition, charging it would plunge the country into chaos. Roh said he would establish diplomatic relations with China if elected.

Another major opposition candidate, Kim Dae Jung, told a news conference that, if elected, he would disband the country's main intelligence agency, the National Security Planning Agency. He said the agency had bribed his secretary to write a book that libeled him with false political charges. He gave no details.