The loud crack of a gunshot startled protesters as they hurled objects and clashed with riot police in Charlotte, North Carolina, where racial tensions soared a day after a black man was killed by officers.

One man near the center of Wednesday night's confrontation was on life support after being shot by another civilian, the city said in a tweet, reversing its earlier statement that the man had died.

The unrest turned violent after a peaceful vigil to mark the previous day's shooting death of African-American Keith Scott at the hands of Charlotte police. 

It was the latest in a long series of controversial police killings of black men which have inflamed tensions in several US cities, and sparked a national debate about race and whether Americans can come together to reduce police brutality.

The Charlotte protesters came face to face with helmeted security forces in riot gear, who massed uptown but were briefly outnumbered by the hundreds of demonstrators who took over the streets, kicking police vehicles and breaking storefront glass.

The police were forced to retreat to the safety of an upscale hotel lobby. Protesters tried to push their way inside but were blocked, and police eventually established a perimeter at the hotel's entrance.

When agitators got too close, or projectiles rained down, police used rubber bullets, pepper spray and flash grenades to disburse the crowd.

Some appeared undaunted, surging back toward the police.

"Your life is in danger, you need to move!" an officer yelled. 

As the protest turned chaotic, a reporter heard the clear sound of a gunshot, which sent several people into a run.

The shooting victim fell to the sidewalk two yards (meters) from an AFP photographer, and riot police and other protesters helped move the injured man behind the police perimeter. A pool of blood was left on the sidewalk.

Wednesday night's mayhem followed a dusk vigil for Scott, a 43-year-old father killed by police in a Charlotte apartment complex under disputed circumstances.

The police chief said Scott was holding a gun and posed a threat to officers, and that the policeman who shot him was black. But neighbors told a dramatically different story, saying Scott was reading a book, not holding a firearm, and that the officer who shot him was white.

- 'It's too much' -

While the vigil was peaceful -- several attendees brought their children -- the atmosphere changed dramatically once demonstrators walked to the nearby police headquarters, where one protester pulled the American flag to the bottom of its flagpole.

Other people banged angrily on the front doors and chanted "No justice, no peace," and "Fuck the police."

By the time they walked the few blocks to uptown, and encountered riot police standing like statues on Trade Street, protesters were seething.

"It's too much. It's too much," winced one woman, wiping tears from her eyes as she stood before riot police.

"We've got brothers and sisters and children and fathers who think we're not going to live to see the next day. Nobody should have to live like that," she said. "Not every black person is a drug dealer, a crackhead, or a gangster."

Several protesters raised their arms as they stood before police, chanting "Hands up, don't shoot."

North Carolina Governor Pat McCrory declared a state of emergency, saying he was bringing in the National Guard to help maintain order.

But while officials are eager to defuse the anger on the streets, many protesters were outraged that the police department has so far refused to release the body-cam and dashboard footage from the officers at the scene of Scott's death.

One woman standing near the riot police held up a sign that read "Release the Tapes."