SEOUL, DEC. 10 -- Rioting jolted one campaign rally for Roh Tae Woo today and forced the cancellation of another as police battled thousands of demonstrators hurling rocks, bottles and tear-gas bombs in protests against the presidential candidate of South Korea's ruling party.

About 180 people reportedly were injured and 42 arrested in the clashes, the most violent outbreak of the country's heated presidential campaign. Roh was not injured.

The rioting occurred in the provincial towns of Kunsan and Chonju. It comes only six days before voters cast their ballots in a tight race among the top three candidates, Roh, Kim Young Sam and Kim Dae Jung.

The rioting had been expected, although on a lesser scale. Roh was staging the rallies in the antigovernment Cholla region, where he has faced varying degrees of campaign violence that apparently involved backers of Kim Dae Jung, Cholla's favorite son.

Political experts said the violence might hurt Kim by increasing sympathy for Roh and painting the opposition as prone to violence.

The main impact of the rioting was to sharpen the edgy atmosphere in Seoul, where the usual flurry of last-minute rumors and maneuvering has begun as the Dec. 16 vote approaches.

Some diplomats have warned vaguely about a possible military coup to interrupt the democratization process before or immediately after the election.

Several sources said, however, that there is no evidence of a coup plot and that the odds for military intervention have not increased significantly in recent weeks.

In a speech last night to the World Affairs Council in Washington, Assistant Secretary of State Gaston Sigur warned that anyone who tries to halt democratization "risks the condemnation of history, the Korean people and the world's democratic community."

Sigur said South Koreans "have shown they are determined" to hold the long-awaited ballot, adding that the United States "cannot foresee any circumstance under which the election would have to be postponed or cancelled."

Today's violence began in Kunsan, about 220 miles south of Seoul. After a speech to thousands of supporters on a Kunsan college campus, Roh started a motorcade in an open-back truck but soon came under attack from hundreds of protesters throwing rocks, bricks, bottles and small home-made bombs apparently composed of firecrackers, news reports said.

Roh was quickly surrounded by security guards who used Plexiglass shields to deflect the barrage. Korean journalists, guards, police and protesters all were among the injured.

Unconfirmed news reports said two youths carrying knives were arrested near Roh, but it was unclear what their intentions were.

Roh was to appear later at a rally in the nearby city of Chonju, but the gathering was called off when thousands of protesters overran the site.

Pro-Roh banners were torn down and burned, and officials waiting for Roh on the speaker's platform were chased away by the rock-throwing protesters, who shouted antigovernment slogans.

Riot police had little luck when they tried to push the protesters back with repeated volleys of tear gas.

Evening broadcasts on state-controlled television showed battlefield-like scenes reminiscent of the June protests that forced Roh and President Chun Doo Hwan to agree to stage the December election.

Riot police were seen battling groups of demonstrators on streets in Kunsan and Chonju that were choked with tear gas and littered with rocks and other debris.

One person with blood flowing from head cuts was seen being led away for medical help. A group of protesters also was shown beating a man who appeared to be a Roh supporter.

At a news conference, Roh blamed the violence on "impure elements" -- a generic label here for radical leftists. In his speech earlier in Kunsan, Roh reportedly vowed to "firmly deal with leftist forces and radical elements."

He accused the opposition of trying to smear the government and ruling party by blaming the election violence on police provocateurs.

As the presidential vote nears, attention is focusing on a series of key rallies to be staged this weekend, notably a Seoul rally for Roh on Saturday.

The event, scheduled for the eighth anniversary of the controversial 1979 coup that brought Chun to power, could be the setting for a significant speech, and perhaps another confrontation with protesters.