A boy steps boldly into the night traffic and waves a gun to bring the cars to a halt, clearing a path for a motorcycle which screeches into the intersection. Riding pillion is another boy, brandishing a machinegun.

Later two teenagers, also riding pillion on motorbikes, flash their guns at other motorists; nearby, a boy can be seen taking aim with a rifle equipped with a telescopic sight. Other youths wander the street smoking crack.

For residents, the junction between the busy Dom Helder Câmara and dos Democráticos, in North Rio de Janeiro, has become known as the Corner of Fear — and video footage of daily life there has shocked a nation already familiar with guns and violence.

The latest images, captured by undercover journalists from the Rio tabloid Extra, have exposed the city’s criminal youth culture in a manner that echoes the journalistic investigation featured in the film City of God.

The age of the criminals — one pistol-toting boy is 12 — is obvious cause for alarm, but so is the seeming impunity with which they act.

The video footage has provided a glimpse into the city’s underworld that hardly touches Rio’s wealthier citizens.

Local newspapers rarely show at first hand the violence that permeates the city’s slums (favelas). Since the brutal torture and murder of the journalist Tim Lopes — who was caught filming secretly in the Vila Cruzeiro favela in 2002 — Brazilian reporters have been reluctant to take their cameras into slum areas. Any reports that are filed tend to come from correspondents talking from inside armoured cars, or are images showing the aftermath of a shooting.

“What is shocking is this parallel power, the fact that they are very young,” said André Cabral De Almeida Cardoso, 41, a teacher. “They are so brazen about it.”

Valera dos Santos, 34, a maid who lives in a favela in São Paulo, said: “My God, I’ve never seen pictures like this. It’s absurd, they’re just boys.”

The journalists who captured the images were also taken aback. “Even knowing the reality of what could happen, you are still shocked by the glamour that these weapons represent in the arms of minors,” said Fernando Torres, 27, one of a team of three who spent four nights undercover at the Corner of Fear.

“These images are desolate,” said Lucy Petroucic, 56, a translator. “These boys have become little Taleban who think they have nothing to lose.”

Within hours, police arrested one of a group of bandits shown in the video and promised that changes were on the way. Luiz Fernando Pezão, Rio’s Deputy Governor, told reporters that a new police base would open nearby in May.