Malcolm X and the Black Panthers believed that black citizens in America should use "any means necessary" in order to achieve equal rights. Malcolm X and the Black Panthers believed that black citizens have been mistreated and physically abused for way too long. They argued that blacks should use self defense as a means to protect themselves from white oppressors.
On the other hand, Dr. Martin Luther King Jr. believed in nonviolent protest. King felt that retaliating with violence would cause more problems than it would solve. This is why King advocated for actions like sit-ins, boycotts, and marches.
The Black Panthers might have been popular in urban areas due to the fact that major cities were places where black citizens constantly saw violence against members of their own race. They felt empowered by this group to take a stand against people who oppressed them.