Thousands of riot police and opponents of the government battled through the afternoon in a shopping and entertainment district of central Seoul today after South Korean authorities banned a rally called to protest alleged torture of prisoners.

The violence shattered a truce established two months ago between the government and the main opposition group, the New Korea Democratic Party, when the government bowed to the party's demands to discuss revision of the constitution.

Senior members of the opposition party were among the crowds that defied the ban today and were tear-gassed. A party vice president suffered minor injuries after being hit by a gas canister and a number of party members were arrested. The government said none was a member of the National Assembly.

Kim Dae Jung, a senior opposition leader, was put under house arrest to stop him from attending the rally.

This afternoon, hundreds of opposition party members began a sit-in at their national headquarters in Seoul to protest the government's actions. The opposition party had reined in its street demonstrations after the constitutional talks began in the assembly.

Today's violence led some members of the opposition to suggest that strategy was wrong. "I think it is time for us to reopen our struggle outside the assembly," Kim said in a telephone interview tonight.

The rally was to have been held in Myongdong Cathedral, archdiocese for South Korea's 2 million Catholics. But the government declared that the true purpose was to "disseminate false rumors . . . thereby misleading and agitating the public." The rally was "likely to greatly set back the progress of democracy now under way in Korea in a calm atmosphere," it said, adding that "some are attempting to obstruct the attainment of the national goal through violence and demagoguery." Opposition rallies here frequently turn violent.

Government officials also said South Korea's Cardinal Stephen Kim had not given permission for use of the cathedral. But opposition leaders said he had. Police arrested a number of the rally's organizers for questioning.

Officials maintain that the government wants to promote democracy but that a threat from communist North Korea necessitates a firm grip on public order. Kim Dae Jung and other critics cite actions like today's as evidence that the government is not sincere about moving away from authoritarianism.

About 2,000 helmeted police and plainclothesmen were bused to the brick Gothic cathedral this morning to seal it off. A roughly equal number of opposition members, many of them students, came anyway.

At one spot, opposition politicians and a Catholic priest strong-armed their way through police lines. At another, students formed a line for a snake dance and attacked police lines with umbrellas, kicks and punches. The police fired tear-gas canisters and charged into the crowd.

Later, other dissidents sat on the pavement in a light drizzle, sang antigovernment songs, tossed leaflets into the air and taunted the police, who stood a few yards away.

Tens of thousands of shoppers who thronged to the fashionable boutiques, theaters and coffee shops of the Myongdong area, meanwhile, also were caught by the gas. They went about their shopping with handkerchiefs over their mouths. Many shop owners shuttered their doors.

Violent student-police battles are an almost daily occurrence in South Korea when universities are in session. Demonstrations in downtown Seoul involving party members, however, are rare.

The Seoul newspaper Jung-ang Ilbo reported that about 12 opposition leaders in addition to Kim Dae Jung were put under house arrest today and 20 party members were arrested on the street. Kim Young Sam, a senior adviser to the party and party president Lee Min Woo were among a crowd that was tear-gassed. Party Vice President Yang Sun Jik was reported to have been struck on the abdomen by a canister and treated at a hospital.

The protests grew from allegations from a 22-year-old woman that she had been sexually abused by a South Korean policeman while being interrogated last month about her alleged role in clandestine labor organizing. The New Korea Democratic Party and other opposition groups took up her cause, forming a lawyers' committee to aid her.

Government prosecutors conducted an investigation and earlier this week announced they had found that during the interrogation, a policemen had struck her on the breasts six or eight times after forcing her to remove her jacket and had used offensive language.

However, the prosecutors concluded that this did not constitute sexual abuse. Her accusation of such was "a routine tactic used by student radicals to promote their ideological cause," the government said, adding that the policeman had been fired.

The opposition denounced the investigation as a whitewash and called the rally at the cathedral to protest the woman's treatment as well as alleged abuse and torture of prisoners in general.

Cardinal Kim was reported to have sent the woman a letter today praising her for courage. Sponsors included the opposition party, Protestant and Catholic clergy, human rights groups and other dissident organizations.

Government officials say torture is rare here. "We'll try to stop such cases of human rights abuses," Choi Chang Yoon, secretary to the president for political affairs, said in an interview yesterday. Some officials say the woman's case is evidence that things once swept under the rug are now being dealt with openly.

The London-based human rights group Amnesty International said in a report this year that it has received numerous reports of physical abuse of prisoners during interrogation but that it knew of only two cases of officials being prosecuted for assaulting prisoners in their custody.