Marielle Franco, 38, was a Rio city councillor for the left-wing Socialism and Liberty Party.
She was shot dead in Rio when a car pulled up alongside hers and two men opened fire from inside the vehicle.
Her killing has caused outrage and protests are planned for Thursday.
'Targeted'
Ms Franco was returning from an event encouraging black women's empowerment when a car drew alongside hers in the northern part of the city and nine shots rang out.
She and her driver, Anderson Pedro Gomes, were both killed, and her press officer, who was sitting in the back seat of the car, was injured.
Police officials said it appeared Ms Franco had been deliberately targeted.
Ms Franco was elected to the city council in 2016 and presided over the women's commission.
Last month, she was chosen to be the speaker of the commission overseeing the deployment of federal security forces into Rio's favelas.
Brazilian President Michel Temer deployed the military in the state of Rio in February after a spike in violent crime during carnival.
The measure was approved by Brazil's Congress but has stirred controversy with some residents complaining about harassment.
Ms Franco, who grew up in Maré, a favela complex in the north of the city, has been an outspoken critic of the move to deploy the army and the federal police force.
On Tuesday she posted on Twitter about the killing in the Jacarezinho favela of a 23-year-old man, which the youth's family blamed on the military police.
"Another killing of a youth which could end up on the PM [military police] tally. Matheus Melo was leaving church. How many more will have to die before this war ends?" she asked.
In another tweet she expressed her solidarity with the people of the favela of Acari.
"What is happening now in Acari is absurd!" she posted. "The 41st battalion of the military police is known as the battalion of death. Enough of trampling all over the population! Enough of killing our youth!" she wrote, adding a picture with the words "We're all Acari, stop killing us!"
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