SEOUL, JUNE 11 -- Riot police firing tear gas today battled several hundred South Korean students holed up at Seoul's main Catholic cathedral, but otherwise the city began returning to normal after an outburst of fierce street violence yesterday.

Students entered the grounds of Myongdong Cathedral last night. This morning new clashes broke out around it, with police firing hundreds of canisters of gas, some of them directly into the church's grounds.

In number of participants, the protests today receded to a size found almost daily on South Korean university campuses. But their location at the center of the capital and the involvement of one of the country's most respected religious institutions gave them unusual political significance.

A number of Catholic priests came to the church and offered moral support to the students.

At one point this evening, priests walked down rubble-strewn streets to police lines to protest the gassing of the cathedral. Priests told reporters they had been negotiating a peaceful withdrawal of the students and an end to the siege when a major attack occurred.

The students did not scatter, however, and the police have not moved in to arrest them, apparently due to churches' informal status as political sanctuaries that police cannot enter without risking censure from the public.

Late tonight, rows of helmeted police were deployed in streets and alleys approaching the cathedral and appeared to be settling in to wait.

Inside, students chanted, "Down with military dictatorship," sang political songs and stockpiled gasoline bombs. They fashioned cloth and plastic gas masks and erected crude barricades that included plywood, tree branches, flowerpots and trash cans.

"Now we will see if we can get democracy or not," said one student in the cathedral.

During the day, they burned an effigy of President Reagan to underscore their contention that the United States props up the government of President Chun Doo Hwan.

Late tonight, police fired dozens of gas grenades in the nearby Namdaemun outdoor market to try to disperse several hundred students who appeared to be attempting to join the people at the cathedral.

Some people here are calling the protests in Seoul and approximately 20 other cities the most extensive since an uprising in Kwangju city in 1980 that left more than 200 people dead.

The national police said today that a total of 3,831 people were detained in protests around the country yesterday and that 708 policemen were injured. Another 301 people were reported detained today. Three police substations were attacked and three police vehicles and two motorcycles were destroyed in yesterday's violence, the police said.

In addition, two students were reported seriously injured yesterday by flying tear gas canisters. A third student who was struck on the head by a canister Tuesday was reportedly being kept alive only through use of respirators.

The government warned today that it would not tolerate further disturbances of the type seen yesterday. But other government statements had a conciliatory tone. Ruling party spokesman Hyun Hong Joo said the protests "tell us that there are many people who simply do not believe what the government is saying."

The demonstrations were originally called to protest the death of a student activist during police torture in January and the ruling party's nomination for president yesterday of Roh Tae Woo, a former military classmate of Chun.

Roh was expected Friday to propose a reopening of dialogue with the opposition. However, there seemed little chance of positive reception from the opposition, as the government continues to reject its demands for immediate revision of the constitution to alter the method by which the president is elected.