A fragile calm was maintained after a night that saw intense exchanges of gunfire, filling the dark sky with bright streaks as bullets whizzed into and out of the Alemao slum complex - an area housing more than 80,000 slum dwellers.
The slum, which is near the highway to the international airport, is known locally as the "Gaza Strip".
The lockdown comes after a week of intense and widespread violence in Rio, with dozens of mass robberies of motorists on key roadways, more than 100 cars and buses set on fire and at least 35 deaths, mostly suspected drug traffickers.
Authorities say the gangs are reacting to a two-year police campaign that has pushed the criminals out of the slums where they have long ruled with impunity.
The state government's clampdown is part of its attempts to secure Rio before the city hosts the finals of the 2014 World Cup and the 2016 Olympics.
Rio de Janeiro's governor, Sergio Cabral, has vowed to break the back of drug gangs that have ruled hundreds of shantytowns - or favelas - in the city of six million people.
But Amnesty International has complained that police have been too heavy-handed.
"The police response has put communities at risk," said Patrick Wilcken, Amnesty International's Brazil researcher. "The authorities must ensure the security and well-being of the broader population comes first and foremost in any operation in residential areas."
Many Rio residents seemed to welcome the police stance, however, applauding as armoured vehicles rolled by and voicing hope they would reclaim areas of their city that had been lawless for years.