For millions of people in the United States and around the world, the truest representatives of the tumult, dreams, and terrors of the 1960s and early ’70s were not hippies, Yippies, or Hells Angels, but the Black Panthers. Founded by Huey Newton and Bobby Seale in October 1966 in Oakland, Calif., the Panthers began as a sort of urban militia, formed to protect African Americans from police brutality. The group grew into a small but influential national movement, espousing black nationalism and self-sufficiency, a sort of pseudo-Maoism, and — controversially — violence against authority figures. (See the gallery, The Black Panthers in Action.) Here, on Bobby Seale’s 75th birthday, LIFE.com offers another, less-militant glimpse of an original Panther and his fellow radicals, in previously unseen photos taken at Seale’s 35th birthday party in 1971. Above: An unpublished photo by LIFE photographer John Olson of former Chicago Seven defendants David Dellinger (left) and Abbie Hoffman (second fr. left) with Bobby Seale, October 1971.