Political cynicism is rife in Brazil, for good reason. Years of scandal have left many regarding elected representatives en masse as corrupt, incompetent liars and democracy as broken. Murder is frequent in Rio de Janeiro. Yet tens of thousands have taken to the streets across the country to protest against the killing of Rio politician Marielle Franco and her driver, in what is widely regarded as an assassination.

Franco, 38, was shot dead on Wednesday, only 18 months after her election to the city council. She was a black single mother from the favelas in a field dominated by privileged white men. Sceptics wondered if she could get elected, yet her tally of votes was the fifth highest of more than 50 councillors. In her short tenure she had become a beacon for progressive politics locally, on issues including LGBT and women’s rights and an outspoken critic of the aggressive, militarised policing of the favelas, where residents live under the brutal control of drug gangs, but are also terrorised by state violence.

Brazil sees a 10th of the world’s homicides. The toll rose to 61,600 in 2016; police killed more than 4,200 people. The victims, in both cases, are disproportionately black. Rightwing President Michel Temer put the army in charge of the city’s security, with one eye on the rise of extreme right former army captain Jair Bolsonaro, who is polling in second place ahead of this year’s presidential elections and has thrived on fomenting fears of disorder. Favela residents have bitterly opposed the move, fearing it will only spur more bloodshed. “How many others will have to die for this war to end?” Franco wrote in one of her last tweets, addressing police violence. She had just been appointed rapporteur for a commission monitoring the militarisation of public security. Police and prosecution sources have indicated that they think it most likely her killing was related to her political work.

The outrage of her murder should only illuminate the power of her message. In some ways her story symbolises the evolution of social movements in Brazil and the way that a longstanding working-class protest movement has begun to form connections with middle-class activism: she was an intellectual born and raised in a favela. Some supporters hope that her death will prove a turning point, saying the strength of the protests suggests it has broken a widespread apathy.

Set against this are the grave fears of Brazil tilting still further away from justice, fairness and security, particularly given the popularity of Mr Bolsonaro. It would be especially punitive for those already hit by austerity and harsh police tactics. In this context, international condemnation of this murder matters. It offers moral support to protesters and reminds mainstream politicians that Brazil will be judged on whether it brings Franco’s killers to justice and listens to her warnings. Authoritarianism will further brutalise poor communities, which have already suffered such violence.