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.

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.

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.

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.

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

- 'It was an execution' -

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.

"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.

- 'All bad guys' -

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.

"When the police leave, the gangs aren't weaker. The traffickers just get more guns and come back more powerful than before," said Silvia Ramos, head of the Security Observatory at Candido Mendes University.

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