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 Rio's policing, 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.
Rio's security situation has been deteriorating rapidly since the city hosted the 2016 Summer Olympics, with well-armed drug gangs making numerous poor favela neighborhoods virtually off limits to police.
However, the trigger for the decision to send in the army appears to have been 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.
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.
There is particular controversy over government calls for the army-led police to be able to serve mass 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.
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