The town's alive this weekend with activities in honor of Martin Luther King's birthday:

* The National Theater is holding two family-oriented Saturday performances at 1 and 4:30. Each features a variety of live entertainment, including a special appearance by Cosby Show star Malcolm Jamal-Warner, who will breakdance with Washington's Mighty Poppalots; music from Broadway singer Marta Heflin; a magic act; singing by the Children's Urban Arts Ensemble; and African, Chinese and contemporary dance. Free tickets are available at the door; it's recommended you arrive at least 30 minutes before showtime. The National Theater is at 1321 Pennsylvania Avenue NW, within easy reach of the Metro Center station and the Federal Triangle station. Call 783-3372.

* "Of Song, Peace and Struggle," a three-hourprogram in recognition of the 1955 Montgomery bus boycott, takes place at the Museum of American History Saturday in the museum's Carmichael Auditorium. At 1, civil rights activist Jamila Jones will discuss her experiences in the boycott; from 2 to 3:30, there'll be a song workshop conducted by original members of the Montgomery Gospel Trio; and between 3:30 and 4:15, you can meet at the information desk to join a tour of museum exhibitions on the history of social change in the United States. The museum is at 14th Street and Constitution Avenue NW. Call 357-2700.

* A gem of a jazz concert, "Free at Last, Free at Last," takes place at 8 this Saturday at the Martin Luther King Memorial Library. The free show features music composed and performed by Archie Shepp and Malcolm Goldstein, along with nine internationally acclaimed musicians, including Denardo Coleman, Borah Bergman, Joseph Celli and Diedre Murray. Seating is on a first-come basis beginning at 7. The library is at Ninth and G streets NW. For more information, call 727-1271.

* Folk singer Pete Seeger joins the Washington Philharmonic this Sunday evening at 6 in a concert combining orchestral pieces, gospel, spirituals and folk songs. The concert will be held at the Metropolitan A.M.E. Church at 1518 M Street NW. Tickets are $10. For reservations, call 699-5440.

* Jeffrey Osborne and Jennifer Holliday will perform a benefit concert for the MLK Mural Project this Sunday at 8:30 at the Warner Theater, 513 13th Street NW. The $25.50, $50 and $100 tax-deductible tickets are available by calling 626-1050.