Violent clashes between protesters wielding rocks and molotov cocktails and riot police using peppery tear gas prevented the latest in a series of antigovernment rallies here today, adding an explosive new element to South Korea's volatile political scene.

Inchon police officials said more than 30 riot policemen were wounded and more than 100 persons arrested in the protests. An unspecified number of demonstrators were also hurt and several cars were turned over or burned.

The clashes came in this industrial port city 20 miles from the capital, during a visit to Seoul by British Prime Minister Margaret Thatcher and three days before a visit by Secretary of State George P. Shultz.

This was to be the seventh and perhaps most important mass rally in the three-month campaign for major constitutional change by the opposition New Korea Democratic Party. The demonstrations of Korean "people power" have been a major force in South Korean politics since they began in February, displaying broad and growing public support and forcing a reluctant President Chun Doo Hwan to agree Wednesday, at least in theory, to consider constitutional change during his remaining 22 months in office.

The violent clashes today suggested a serious split between moderate, consensus-seeking opposition politicians and militant student and labor groups brandishing anti-American slogans along with their rocks and kerosene-filled bottles.

Besides making it impossible to hold the rally, for the first time since the current series of protests began, today's violence seemed to validate government charges that the opposition movement was leading to civil disorder. The government party immediately took advantage of the violence to issue public statements geared to this Confucian society where order is highly valued and where the presence of 40,000 U.S. troops is seen as essential insurance against attack from North Korea.

Opposition leader Kim Young Sam blamed the government for the violence, charging that a "premeditated and intentional obstruction" by police had prevented the rally from taking place. Kim declared that riot police had "made unnecessary tear gas attacks against students on the streets" and thus were responsible for the fiasco in which thousands of bystanders were tear-gassed as they waited in vain for the start of the scheduled political rally.

Independent observers on the scene, however, said the violence seemed to have been started by students and workers who attacked an office of the government party a few blocks from the rally site with bricks, rocks and molotov cocktails.

Riot police drove back the attackers, who responded by wielding their weapons against the police and burning an automobile. Police then brought up an armored vehicle that spewed thick clouds of "pepper fog," a highly spiced variety of tear gas, which produced coughing fits among citizens awaiting the rally in the public hall as well as among the students outside.

Conflict between the moderate opposition and more radical students surfaced Tuesday when seven major opposition leaders held a press conference decrying "radical leftist ideas" such as anti-Americanism in some student groups. Opposition leader Kim Dae Jung, who is often considered the most militant of the conventional antigovernment political leaders, was the chief spokesman at the press conference.

Many of the demonstrators at today's rally criticized the main opposition parties as being ready to compromise with the government.

Police blockaded Kim Dae Jung's home in Seoul today, preventing him from traveling to the Inchon rally, as he has been blocked from attending other rallies in the series. His fellow opposition leaders made it to Inchon but were unable to reach the hall because of the fighting outside.

Following Chun's concession Wednesday, which for the first time contemplated a constitutional change that would affect the election of his successor, the government party asked the opposition to end its rallies, which have provided much pressure for a constitutional amendment.

The opposition refused, declaring that Chun's willingness to consider a constitutional change fell short of meeting its demand for direct presidential election.

After today's events, however, the future of the mass rallies is in doubt.