SEOUL, JUNE 15 -- It was a handful of students who began the rally that took place shortly after noon today outside Seoul's Myongdong Cathedral. But within half an hour, thousands of solidly middle-class office workers, lunch-time strollers and shoppers had joined in, blocking the street with a high-spirited festival of songs and antigovernment slogans.

Women leaned from high-rise buildings to shower roses and confetti on the crowd. People talked spontaneously to strangers. A man stood atop a stool and led the crowd in cheering, thousands of fists stabbing the air with each round.

"Initially people are afraid," said a trading company employe in suit and tie. "But when they get together, there is no fear."

Until this week, common belief in South Korea was that the middle class could never make common cause with the rock-throwing radicals of the campuses. Five days of demonstrations here have drawn that wisdom into question and could rewrite the rules of politics here.

Invariably there are comparisons to the "people's power" revolution in the Philippines last year.

So far, the numbers of people in the streets are tiny compared to those who toppled Ferdinand Marcos in Manila. The turnout near the cathedral this afternoon, for example, was probably statistically insignificant in a city of almost 10 million inhabitants.

Yet, the public participation is raising talk of a new mood prevailing in South Korea, in which more people may act on long-repressed hostility toward the government of President Chun Doo Hwan.

As an Army general, Chun seized power in stages starting in 1979. Many South Koreans never have accepted his legitimacy as president, especially in view of the deaths of more than 200 people in demonstrations in the city of Kwangju the following year.

The imperial airs that the short, balding man affects add to public hostility. Koreans appear not to mind such behavior if they feel the leader has earned the right to office. But in many people's minds, Chun has not.

South Korea's paradox is that while development of its political institutions has remained stifled, its economy has raced ahead into the industrial age. Gross national product has roughly doubled since Chun came to power.

Chun is not credited for any of this by ordinary people. But it has long been said that as people acquire houses, automobiles and prospects for promotion in thriving companies, they will be less inclined to risk it all by taking to the streets for political protest.

Now, a confluence of traumatic events seems to be pushing many people to do it anyway.

In January, a student died during police torture, and the extent of official involvement in his death was covered up. In April, Chun suspended debate with the opposition on constitutional reform, ending a year-long period of cautious but nonetheless unusual optimism for a long-term political settlement.

Last week, the ruling party formally nominated for president another former general who helped Chun carry out a coup d'etat in 1979, Roh Tae Woo. Chun has presented this first "peaceful transfer of power" in South Korean history as a monumental event.

But many South Koreans see it as simply replacing one dictator with another. "The bald man with a wig," is what some of them are calling Roh, signifying he is just Chun in disguise.

Public anger has also been stirred by indiscriminate tear gassing by police during the protests of the past six days. The fumes have settled over thousands of homes and workplaces, choking children walking to school and people heading for markets. A press campaign against it is under way.

Police battling with students on the streets repeatedly have had to contend with anger and insults from onlookers.

Radical students who took refuge in Myongdong Cathedral last week were showered with provisions of food, drink, first-aid equipment and clothing. Cash donated has totaled about $25,000 so far, according to press reports.

Public anger also rose against a popular comedian named Kim Pyong Jo for a joke he made while entertaining the crowd at the ruling party's convention. It was a play on words to the effect that the ruling party gives love to the people while the opposition gives pain.

To apologize, Kim has withdrawn from television appearances and a comedy show that he hosts. Ice cream commercials in which he appears are being pulled off the air.

Increasingly in recent days, ordinary people are coming out openly to demonstrate. When they do, they may be radicalized by the police's response.

Today's demonstration at Myongdong was peaceful but ended abruptly when, without warning, squads of policemen in helmets and gas masks sprinted from around corners and, hurling tear gas grenades, charged the crowd. Women screamed. The crowd surged away, some members darting into open shop fronts. Some people were overcome by the gas.

Despite the changes, many of Chun's opponents remain unwilling to move and indecisive about joining the protests.