Authorities in Brazil say 18 people have been killed during a raid against a criminal gang that controls one of Rio de Janeiro's most violent favelas.

Four hundred heavily-armed military police were deployed to the Alemão favela in the early hours of Thursday, July 21.

Sixteen of the dead were suspected criminals, while a police officer and a bystander were the other two victims, officials said.

The operation lasted all day and left thousands trapped in their homes. The objective of the raid was to locate and arrest criminals who were planning operations in rival slums, police said.

Some of the targets were wearing uniforms similar to military police, which made them harder to spot, local media outlet O Dia reported.

Ivan Blas, spokesperson for the state’s military police, said at a press conference that the number of dead is still being tallied. In earlier statements, the force said that an officer was killed, and that the raid had targeted a criminal group in Complexo do Alemao that stole vehicles and cargo and robbed banks as well as invaded nearby neighborhoods.

"I will continue to fight crime with all my strength. We will not back down from the mission of guaranteeing peace and security to the people of our state,” said Rio state Gov. Cláudio Castro on his official Twitter profile, lamenting the officer's death.

But many disagree with the government's strategy for tackling violence and organized crime, an approach that regularly sees deadly police operations.

“ENOUGH of this genocidal policy, governor!” Talíria Petrone, a federal lawmaker for Rio, said in response to the governor's tweet. “This failed public security policy leaves residents and police on the ground, en masse. It’s no longer possible to keep piling up black bodies and favela residents every day!”

Robert Muggah, co-founder of Igarapé Institute, a Rio de Janeiro-based think tank focused on security, said Thursday's raid is "a symptom of failed leadership and an institutional culture that condones excessive force."

"The killings resulting from large scale police operations is a grim reminder that militarized policing is not only ineffective, it is counterproductive," Muggah said in a text message, adding those raids generate "extreme violence predominantly affecting low-income Black populations while also corroding the trust between residents and law enforcement."