TAEJON, SOUTH KOREA, JUNE 17 -- Street vendors working the center of this shady city of 900,000 people know that business can't last much past 6 p.m. these days. By then, the students are sure to be massing on the avenue that leads to the provincial government buildings. Rocks will be flying and the evening air will be choked with tear gas.

Fierce demonstrations this past week in the South Korean capital of Seoul have received worldwide attention. With less publicity but comparable import, major protests have exploded in many of the country's provincial cities, bringing the antigovernment fight to a new stage there, too.

Tonight, police and demonstrators again transformed the city's main avenue into a war zone. Major protests were also reported in Pusan, South Korea's second largest city, and Chinju.

The patterns seen here this past week have been much like those in Seoul. Most of the demonstrators are students. The action is in the public eye on city streets rather than on secluded campuses, and some ordinary citizens are showing support.

On Monday, an estimated 5,000 to 6,000 students from local universities paraded peacefully through the streets to demand an end to the government of President Chun Doo Hwan.

One Taejon woman said onlookers tossed cigarettes, soft drinks and rolls into the crowd, while others shouted encouragement.

"Before, people did not like the students," she said. "That has changed in the last few days."

Taejon has never been a stronghold for dissidents or protests. As visitors quickly hear, people here feel they have a special character of peace and contemplation, said to be a holdover from the yang ban aristocratic class of premodern times.

Located about 100 miles south of Seoul, Taejon is known as the "center city" of South Korea, the place where the main highway splits into two on the drive south.

Like most South Korean cities, this one is growing prosperous on export-oriented industry. It has 340 factories. Education is a big business, too, with eight colleges and universities with about 53,000 students.

But good times in the economy do not generally translate into support for the government. Many people feel that their government is a brutal military dictatorship that should be removed.

Yesterday, two local offices of Chun's ruling Democratic Justice Party lost window panes to hit-and-run attacks with stones.

Government officials said few citizens are willing to use force. They described the violence as orchestrated by dangerous radicals who are trying to overthrow the entire social and legal order.

"I can understand people attacking the party office and breaking our windows," said Yun Chung Won, a ruling party campaign manager. "But these people have even attacked ward offices and police stations."

The current protests began on June 10, the same day demonstrations started in Seoul. Here as in Seoul, an accumulation of grievances seems finally to have come to a boil: the torture death of a student by police, Chun's cancelation of talks with the opposition toward constitutional reform, his nomination of a military classmate to succeed him, tear gas.

Today's action began on the city's campuses. Under a copse of trees at Hannam University at 2 p.m., about 200 students listened to speeches and examined helmets, gas masks and a tear-gas rifle that had been captured from police in previous days' fighting.

Soon about 50 student "commandos" carrying gasoline bombs were marching through the front gate toward a line of police on a bridge. Within minutes, the pock-pock of gas cannisters exploding was heard as the police charged. The students fell back.

The real action, however, began in town at about 6 p.m. Thousands of students sneaked around police lines at their campuses and began massing on corners on Taejon's main commercial avenue. Soon they were spilling into the streets, blocking traffic.

Now it was the police's turn to attack first. Hundreds of them with helmets and shields formed lines and began advancing into the crowd. Suddenly they let fly with a barrage of gas. Many of thecanisters were hurled at head level like rocks.

With that, a full-scale riot began. Students with gauze and plastic over their faces regrouped and counterattacked with rocks and occasional firebombs. A shelter for traffic police was burned down, putting a haze of black smoke over the area.

People watched in fascination from coffee shops, night schools and roof gardens. There also was an element of the routine, however. Two mothers leaving a gas-filled building with four girls seemed to be treating it not as a nightmare but as an adventure.

The street vendors were long gone. Before the fight broke out, one of them said he hated demonstrations. Another agreed it was horrible to lose so much money. "But that doesn't mean we oppose the purpose of the rallies," he added.

United Press International reported the following from Seoul.

The government urged colleges across South Korea to close as protesters burned buses, hijacked a train and a tanker truck and blocked streets around the U.S. Consulate in Pusan.

The national police chief ordered 120,000 troops to remain on red alert while political bickering between the ruling and opposition parties stalled attempts to find ways to end the nationwide violence.

About 45,000 students took part in protests in several cities, news reports said.

The U.S. Embassy in Seoul issued a warning on Armed Forces Television telling Americans to keep away from the U.S. Consulate, train station and International Market in Pusan, 205 miles southeast of the capital.

The consulate was ringed by more than 400 riot police sent to prevent about 5,000 student marchers from approaching the facility. The demonstrators, who sealed off the consulate for several hours, were trying to march to a downtown Catholic center in support of 300 students staging a sit-in, an official said.

In Chinju, 180 miles south of Seoul, 100 students hijacked a truck filled with liquefied propane gas. Police retook the truck in a cloud of tear gas, but 300 students then hijacked a train until they were forced to retreat by police.

In Washington, a State Department official said the administration would not oppose a resolution in Congress calling on Chun to take steps toward a more democratic government.