South Korean opposition leader Kim Dae Jung is a practitioner of "demagogic politics" who will face imprisonment for sedition if he returns home from exile in Washington as he plans to do early next year, the South Korean government has told Sen. Edward M. Kennedy (D-Mass.).

A strongly worded letter from South Korean Ambassador Byong Hion Lew, released by Kennedy yesterday, said Kim "is required by law to serve the remainder" of his 20-year prison sentence should he return home. Kim served 2 1/2 years of the prison term before being put aboard a plane to the United States on health and humanitarian grounds late in 1982.

Lew's letter suggested that the government of President Chun Doo Hwan, which Kim refers to as a military dictatorship, has lost none of its antipathy for its best known political opponent. The ambassador charged Kim with "selfish political statements and plans" and "dangerous agitation and incitement to violence" following the assassination in 1979 of President Park Chung Hee, a period when Chun was maneuvering to seize power in a military showdown.

Lew went on to charge Kim with violating a pledge not to engage in political activities in the United States, with staging demonstrations and spreading "false, distorted and exaggerated views" of the political and human rights situation in South Korea.

Kennedy replied to the ambassador that Kim apparently is being treated more harshly than others convicted with him in what was widely described as a political prosecution. Kennedy called Kim "a moderate, democratic leader" and said it appears he is being punished "merely for exercising the fundamental right of free speech in his home country."