Boston — As expected, the immediate aftermath of Sunday's showing of the television movie ''The Day After'' included mixed reactions: Reagan administration representatives defending United States weapons policies; natural scientists saying the movie actually underrepresented the dangers and impact of nuclear war; citizen vigils and protests; and boasts by ABC that an audience approaching a record 100 million viewed the telecast.
A spokesman for ABC-TV said the network received 1,075 calls immediately after the program was aired: 662 supportive of the movie, 393 negative, and 20 asking for further information about the film.
The program depicted in graphic, ominous detail the effects of a full-scale nuclear attack on Lawrence, Kan.
President Reagan viewed the film last week and commended administration defense policies as the best means for preventing such a holocaust from happening. Later, a White House spokesman said the President ''welcomes the dialogue'' on nuclear policy that the film will generate.
In a special ABC program following the movie, Secretary of State George P. Shultz said the movie portrayed nuclear war as unacceptable, adding his praise for administration policies designed to foreclose on the use of nuclear weapons.
''The moral imperative we feel is felt by others throughout the world, including the Soviet Union,'' Mr. Shultz said, '' and if we are persistent and the propositions are reasonable, we will eventually get somewhere.''
Kenneth Adelman, director of the Arms Control and Disarmament Agency, called the movie ''a very powerful piece that serves as a reminder to all people about . . . the unmentionable horror of nuclear war.''
Mr. Adelman said he did not think there was ''anxiety in the administration on this film. The President has done an enormous amount in arms control.''
News service reports indicated that citizen groups, community and religious organizations, and students throughout the country met to watch and discuss the controversial movie. At the Massachusetts Institute of Technology, four rooms were set up for group viewing. At the University of Kansas in Lawrence, the community targeted in the theoretical attack, students held a candlelight ceremony following the telecast.
Groups favoring and decrying nuclear weapons policies scheduled news conferences, forums and appearances on television talk shows.
In Washington, meanwhile, Cornell University astronomer Carl Sagan repeated warnings that the consequences of nuclear holocaust have been ''underestimated.'' He warned that ''even small nuclear wars can have disastrous climatic effects, enough to generate an epoch of cold and dark.''
At the Asbury United Methodist Church in Kansas City, Kan., more than 100 people joined a group discussion after the movie, UPI reported, many visibly shaken about seeing the movie destruction of their home town, United Press International reported.
The Rev. Bud Cooper told them, ''We must make the world safe for the children. This is an adult issue.''