Despite criticism that the military intervention could lead to violations of constitutional rights, while also failing to address the causes of the urban violence, the Senate voted late Tuesday by 55 to 13, with one abstention, in favor.

Center-right President Michel Temer issued the decree, which put the army in charge of all policing in Rio on Friday. While regular police are still doing most of the work on the streets, generals are now in command, with troops available as back-up.

While the military is in charge of security for the whole state, their focus is on the city of Rio's crime ridden poor favela neighborhoods.

Security in Rio de Janeiro has been deteriorating rapidly since the city hosted the 2016 Summer Olympics, with well-armed drug gangs making numerous favelas virtually off limits to police.

The decision to send in the army appears to have been triggered by a wave of street robberies during the recent carnival festivities that were given heavy media coverage.

"When we see delinquents holding up a street stall with a rifle we can see how serious the situation is. The population lives in terror," said Senator Eduardo Lopes, who led the Senate support for the decree.

Principal opposition came from the leftist Workers' Party.

- Poorly defined mission? -

The military intervention is the first of its kind since Brazil returned to democracy in 1985 after two decades of dictatorship.

Although many support a crackdown, critics worry that the mission has been poorly defined and could lead to abuses.

There are also criticisms that years of heavy-handed policing in the favelas, including numerous army-backed operations, have done nothing to resolve problems rooted in poor education and stunning inequality.

"The intervention in Rio is an inadequate and extreme measure that is worrying because it puts the population's human rights at risk," said Jurema Werneck, head of Amnesty International in Brazil.

Previous use of soldiers in Rio to support the police "did not guarantee an improvement in the violence levels" yet led to "serious violations of human rights," Werneck said.

There is particular controversy over government calls for the army-led police to be able to serve search and arrest warrants in favelas.

One high-profile critic of the proposed measure is star prosecutor Deltan Dallagnol, who spearheads Brazil's enormous anti-corruption operation dubbed Car Wash.

"The penal code does not authorize serving collective or generic search warrants. On the contrary, it demands the greatest possible precision in the homes being searched," he tweeted.