Marielle Franco, of the left-wing Socialism and Liberty Party (PSOL), was assaulted while traveling by car in downtown Rio, the police said.
Unknown gunmen opened fire after bringing their vehicle up close to her car and then fled the scene. The killing is being investigated by the homicide department, while no one has been arrested so far.
Her party issued a statement praising the slaughtered woman for her contribution as an advocate for women's rights in the Rio municipal council and demanding an "immediate and rigorous investigation of this heinous crime."