Police in Brazil faced outraged protests and a UN call for an investigation Friday after a raid on a Rio de Janeiro favela left 28 people dead -- some reportedly killed in cold blood.

"Stop killing us!" said hundreds of protesters from the impoverished neighborhood of Jacarezinho, who marched outside police headquarters accusing officers of perpetrating a "massacre."

"This is one of the most barbaric acts in the history of the Rio police," student Roger Denis told AFP.

Residents protest after a police operation against alleged drug traffickers at the Jacarezinho favela in Rio de Janeiro state Residents protest after a police operation against alleged drug traffickers at the Jacarezinho favela in Rio de Janeiro state Photo: AFP / Mauro Pimentel

It was one of several protests against Thursday's police operation, which rights groups said was the deadliest ever in a city all too used to violence and police killings -- particularly in the poor, majority-black favelas, or slums.

Police said the operation targeted a drug gang that was recruiting children and teenagers. It turned the northern neighborhood into a war zone Thursday morning, leaving streets strewn with bodies and pools of blood.

The initial death toll of 25 -- one policeman shot in the head and 24 "criminals" -- was raised to 28 on Friday, following a funeral for the officer, with a cortege of some 300 police cars blaring their sirens in an emotional tribute.

IMAGES AND SOUNDBITESPeople protest in Jacarezinho, a favela in Rio de Janeiro, a day after a massive police operation against drug traffickers left 25 people dead. IMAGES AND SOUNDBITESPeople protest in Jacarezinho, a favela in Rio de Janeiro, a day after a massive police operation against drug traffickers left 25 people dead. Photo: AFPTV / Bruno KAIKUA

Police now face mounting calls to prove that indeed those killed were "criminals," as well as questions on why the suspects were killed rather than arrested.

Authorities have not released the identities of those killed.

United Nations human rights spokesman Rupert Colville told reporters in Geneva his office was "deeply disturbed" by the killings.

Civil Police officers take part in an operation against alleged drug traffickers at the Jacarezinho favela in Rio de Janeiro, Brazil, on May 06, 2021 Civil Police officers take part in an operation against alleged drug traffickers at the Jacarezinho favela in Rio de Janeiro, Brazil, on May 06, 2021 Photo: AFP / MAURO PIMENTEL

He called for prosecutors to open an "independent, thorough and impartial investigation."

"We remind the Brazilian authorities that the use of force should be applied only when strictly necessary," he said.

Brazilian Supreme Court Justice Edson Fachin ordered the federal and Rio de Janeiro state prosecutors-general to investigate two videos circulating on social media that appeared to show police killing indiscriminately.

One resident said her husband had gone out to buy bread when police shot him at point-blank range
Dos Santos, who worked in construction and at a pizzeria, was not involved in crime, she said.
Witnesses told her he was wounded in the leg when police ope One resident said her husband had gone out to buy bread when police shot him at point-blank range Dos Santos, who worked in construction and at a pizzeria, was not involved in crime, she said. Witnesses told her he was wounded in the leg when police opened fire in an alley crowded with people trying to flee. Photo: AFP / MAURO PIMENTEL

"The events reported appear extremely serious. In one video, there is evidence of acts that, in theory, could constitute an arbitrary execution," he wrote.

Residents described a heavy firefight, while television images showed armed men on rooftops with what looked like high-powered rifles Residents described a heavy firefight, while television images showed armed men on rooftops with what looked like high-powered rifles Photo: AFP / MAURO PIMENTEL

The police deny wrongdoing and say officers followed all protocols, opening fire only to defend themselves. They displayed large piles of drugs and guns seized in the operation.

But accounts from witnesses and relatives raised doubts.

One resident told AFP a wounded young man fled into her home, only for police to follow his blood tracks up the stairs to her second-floor apartment.

Map locating Rio de Janeiro in Brazil, where a massive police operation against drug traffickers in a favela left 25 dead, including one police officer Map locating Rio de Janeiro in Brazil, where a massive police operation against drug traffickers in a favela left 25 dead, including one police officer Photo: AFP / Nicolas RAMALLO

"I just had time to pull my children behind me before they murdered him," she said.

"They didn't even give him time to say anything."

Another resident told news site UOL her husband, 32-year-old Jonas do Carmo dos Santos, had gone out to buy bread when police killed him at point-blank range.

Dos Santos, who worked in construction and at a pizzeria, was not involved in crime, she said.

"It was an execution," she said.

He left behind a newborn son.

President Jair Bolsonaro's inner circle was quick to rally around the police.

The far-right leader won office in 2018 on a pro-gun, pro-security platform, and has close ties with the police and military.

"They were all bad guys," said Vice President Hamilton Mourao, an army general, referring to the suspects killed.

Bolsonaro's son Eduardo, a congressman, criticized "bums" who questioned the police.

"There's a police officer murdered, and they defend the murderers," he wrote on Twitter.

Rio, an iconic beach city of 6.7 million people, is notorious for its violent crime, and also a troubled history of police killings.

Last year, 1,245 people were killed by police in Rio state -- more, for example, than the 1,127 such cases across the entire United States.

Police are rarely held to account for those incidents.

Experts questioned why the authorities continue a highly militarized strategy against organized crime that has produced decades of high death tolls and few results.

"Then the police return shooting even more, leaving the population even more traumatized."