SEOUL, JUNE 26 -- Demonstrators took to the streets in cities around South Korea tonight, defying strong tear gas and mammoth formations of riot police to intensify their protest against President Chun Doo Hwan.

Police fired countless canisters of gas at peaceful and violent demonstrators alike, turning sections of the cities into chaotic battlegrounds.

The protests were called by a coalition of dissidents to bring new pressure on Chun following several days of relative quiet during failed political negotiations.

The demonstrators, mostly students, were attempting to restore the momentum of their street movement, which in the past 2 1/2 weeks has turned into the greatest challenge Chun's government has ever faced.

The size of the turnout indicated that they will be able to keep the protests going in coming days. What remains unclear for the moment is whether middle class people and others who are not professional activists will continue to support the demonstrations. Their attitude is considered crucial to the ultimate outcome of the protests and the fate of Chun's government.

Kim Young Sam, president of the main opposition party, tonight declared the demonstrations a success. "People throughout the country . . . joined us to fight for our cause, to build a democratic country," he told reporters.

Organizers had promised that the demonstrators would not use violence. But Kim said some had been forced into it. "They had no choice but to fight against these militaristic, brutal policemen," he said.

Kim himself had little chance to take part. He and other opposition politicians were roughly hustled into a police van and driven off minutes after they emerged from their party office to join the rally.

They were released several hours later after being driven long distances around the city. Kim's hand was bruised during the encounter. Tonight, an all-night vigil was under way at party headquarters to protest the police behavior.

The party's other major leader, Kim Dae Jung, also was forcibly kept from participating. Police placed him under apparently temporary house arrest this morning to prevent his attendance, less than two days after he had been freed from a 2 1/2-month home detention.

In Seoul, tens of thousands of people appeared to have taken part. It was one of the biggest turn-outs since the chain of demonstrations against Chun erupted.

Protests were reported to have occurred in more than 30 cities. The semiofficial Yonhap News Agency said that in the southern port of Pusan, South Korea's second largest city, 10,000 people turned out, while 20,000 took part in the southern city of Kwangju.

{Press reports early Saturday in Seoul said police had arrested 2,960 people nationwide on Friday.}

In Seoul, the demonstrations unfolded as a series of small ones staged in many different places. Some had a distinctly festive, nonviolent mood.

Up and down the city's Chongno Avenue thousands of people gathered peacefully on sidewalks. They waved handkerchiefs and paper Korean flags, chanted antigovernment slogans and appeals for nonviolence.

Passing motorists honked horns to show support, drawing applause. Drivers of municipal buses often joined in. People scattered when police charged in with tear gas, then cheered when they withdrew to their stand-by positions.

As has often happened, passers-by assailed the police for their use of tear gas. "They don't care about the people," shouted a laborer who was pushing a wooden cart.

At one point this evening, several demonstrators were preparing to stone a contingent of 100 policemen near the city's East Gate. Another protester, however, placed himself in front of them and pleaded with them to drop the stones.

Elsewhere, the mood was unanimous for a fight. Demonstrators arrived with supplies of gasoline bombs and quickly began breaking up sections of pavement for throwing.

Thousands of young people, most of them apparently university students, massed around Seoul's main rail station and fought running battles with riot police late into the night.

At one point they blocked all 10 lanes of an avenue leading to the station. Police cleared it by marching hundreds abreast in a line down the avenue, an armored car with rapid-fire gas launcher in the center.

Police today were unusually quick to fire tear gas and break up even small crowds. In several places around town, they used rapid-fire gas cannon mounted in armored cars.

Authorities were reported to have put about 230 dissidents under house arrest today. Police yesterday declared the demonstrations illegal and said they would be broken up by force.

President Chun, meanwhile, today met with Buddhist leaders as part of a sampling of opinion he reportedly is taking to help resolve the current political crisis, which was triggered by his April decision to suspend talks on constitutional reform.

Chun Wednesday offered to reopen those talks, but the opposition is saying the offer has little meaning.