An apprehensive Reagan administration called yesterday for South Korea to avoid violence and use "political solutions, not military intervention" on the eve of mass rallies that could be critical to determining the country's immediate future.

White House and State Department spokesmen, in identical statements, said that "we regard peaceful demonstrations as a legitimate political statement." They added, "However, we strongly urge all the parties involved to avoid violence."

Official sources said assurances were received from the Korean government that Friday's demonstrations will be handled "in the normal way," which means with riot police rather than any troops. Nonetheless, there was concern in Washington about what could happen when large numbers of Koreans, including radical students, take to the streets amid a volatile political situation.

"Much hinges on what happens in the next 48 hours," said Undersecretary of State Edward J. Derwinski, who was back in his office yesterday after lengthy discussions in Seoul earlier in the week.

Secretary of State George P. Shultz, who returned yesterday from nearly three weeks in Europe and Asia, and Assistant Secretary of State Gaston J. Sigur, who was to return last night from a mission to Seoul, plan to brief President Reagan today on the Korean situation, official spokesmen said.

The continuing U.S. public declarations on Korea and high-profile briefings for Reagan underscored the urgency that pervades administration deliberations on the highly uncertain situation there.

Derwinski said yesterday that he was assured by South Korean Defense Minister Lee Ki Baek that "we do not wish or intend to impose martial law." Speaking of the coming demonstrations, the official said, "I didn't talk to anyone who I thought was responsible who thought this would end with the military having to move."

Derwinski said the most widely discussed compromise solution to the current crisis, which was discussed with him by political leaders representing the government as well as opposition parties, would involve naming an interim president to lead South Korea for a limited term of one year after the promised resignation of President Chun Doo Hwan next February.

Under this scenario, the interim government would serve through the period of the 1988 summer Olympics in Seoul, with a national election to choose a longer-serving leader taking place later next year. A key question, not yet resolved, is whether that successor government would be selected by direct election or indirectly.

Derwinski and others said the U.S. administration has no position on the interim-government idea, considering it to be the sort of issue that must be settled by the Koreans themselves. But the officials did not disguise their interest in it as a possible solution to the crisis.

Shultz, speaking to reporters aboard his plane en route to Washington, urged the Korean opposition to continue talking to the government about election reform, according to Reuter. "I hope what has started in South Korea marks the beginning of dialogue," Shultz said.

The House Foreign Affairs Committee, in a related action, unanimously approved and sent to the floor a resolution urging South Korea to "take concrete and meaningful steps toward democratization." Chairman Dante B. Fascell (D-Fla.) commended Chun for releasing opposition leader Kim Dae Jung from house arrest and said the talks between the government and the opposition in Seoul mean "that the democratization process has hopefully started."