Some of these troops have already been deployed, with transport trucks seen rolling through the city of Rio on Friday afternoon.
President Michel Temer signed a decree allowing the use of the armed forces in Rio de Janeiro state, the official government gazette announced.
Temer plans to mobilize 10,000 soldiers altogether.
The operation aims in particular to fight organized crime in this city of 6.5 million people, Defense Minister Raul Jungmann told a news conference.
He said other countries also need to cooperate in fighting such crime at the regional level.
A year after hosting the Summer Olympics, Rio has descended into a chaos of violence, with an average of three people killed each day by stray bullets in the first six months of the year, especially when there are shootouts between police and drug traffickers.
Around 90 members of the security forces have been killed so far this year in Rio.
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