RIO DE JANEIRO (AFP) - Police in Brazil faced outraged protests and a UN call for an investigation on Friday (May 7) after a raid on a Rio de Janeiro favela left 28 people dead - some reportedly killed in cold blood.

The bodies of three more victims removed from the favela on Friday were men with links to organized crime, according to police. Twenty-four other people and a police officer also died in the operation in the northern Rio neighborhood of Jacarezinho.

"Intelligence confirmed that the dead were drug dealers. They fired at officers, to kill. They had orders to confront," Civil Police chief Allan Turnowski told reporters.

"Stop killing us!" said hundreds of protesters from the impoverished neighbourhood 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. "We're here to ask for justice for this unjustifiable massacre."

It was one of several protests planned 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 neighbourhood into a war zone on Thursday morning, leaving streets strewn with corpses and pools of blood.

One of the dead was a policeman shot in the head, whose funeral on Friday afternoon was expected to draw emotional tributes from colleagues, as well as security hawks such as President Jair Bolsonaro and his inner circle.

The other 27 dead were "criminals," according to police - who now face mounting calls to prove that, as well as questions on why they were killed rather than arrested.

United Nations rights office spokesman Rupert Colville told reporters in Geneva his office was "deeply disturbed" by the killings, and 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. "Lethal force should be used as a last resort."

Residents described a heavy firefight, and aerial TV footage from the raid showed armed men on rooftops passing what looked like high-powered rifles from hand to hand.

Police said officers had followed all protocols before opening fire, and displayed large piles of drugs and guns seized in the operation.

'They executed him'

But accounts from witnesses, relatives and video posted on social media 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 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.

Then police "came right up to him and executed him," she said. "It was an execution. They came to kill."

He left behind a newborn son, she said.

'All bad guys'

Bolsonaro's camp was quick to rally around the police.

The far-right president 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 28 killed.

Bolsonaro's son Eduardo, a congressman, criticised "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 de Janeiro state - more, for example, than the 1,127 such cases across the entire United States.

Experts questioned why the authorities continue a highly militarized strategy against organised 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 traumatised."