National Arts Week in the District of Columbia will be launched tonight in the Pension Building with a gala celebration of the fourth annual Mayor's Arts Awards. The nationwide observance, running through Sunday, launches a yearlong celebration of the 20th anniversary of the National Endowment for the Arts.

With Rene'e Poussaint as master of ceremonies, the mayor's gala will include entertainment, a special 20th-anniversary tribute to the NEA and the announcement of winners, chosen from 27 finalists, in three categories: Excellence in an Artistic Discipline, Outstanding Emerging Artist and Excellence in Service to the Arts.

Nancy Reagan is the honorary chairman of the national 20th-anniversary committee. Charlton Heston, the working chairman, will be featured at the Kennedy Center Sunday afternoon in the climactic event of National Arts Week in Washington. Heston will read Aaron Copland's "A Lincoln Portrait," with the Colorado Philharmonic Orchestra conducted by Carl Topilow. Among the other works on the all-American program will be Billy Taylor's Jazz Suite for Piano and Orchestra, with Taylor as piano soloist.

Visual arts exhibits associated with National Arts Week are being sponsored by many Washington museums, galleries and other exhibition spaces, including Tomorrow's World Art Center, the Corcoran Gallery, the National Museum of American Art, the Renwick Gallery, Washington Project for the Arts, the National Museum of African Art, the National Council on the Aging, the Phillips Collection and the American Federation of Arts.

Performing arts events associated with the celebration include performances of the Erick Hawkins Dance Company at the Kennedy Center; a Kennedy Center recital Friday by Jose' Feghali, winner of this year's Van Cliburn Piano Competition; Andrew Rangell's recital next Sunday at the Phillips Collection; and the opening of the New York City Ballet's Washington season, Saturday night at the Kennedy Center.

Also scheduled as observances of National Arts Week are two national conferences: a National Conference on Black Music Research and a conference of the American Council for the Arts. The National Institute for Music Theater, headquartered at the Kennedy Center, is devoting its fall newsletter to National Arts Week.