The US Justice Department said it would investigate the killing of a black man pinned to the ground and shot in the chest by two white police officers outside a convenience store in Baton Rouge, Louisiana.

Graphic video images of Tuesday's shooting of Alton Sterling, 37, unleashed protests and social media outcry on Wednesday over the latest alleged police brutality against African-Americans in cities from Ferguson, Missouri, to Baltimore and New York.

One officer shot Sterling five times at close range, and the other took something from his pocket as he was dying, according to images recorded by Abdullah Muflahi​, owner of the Triple S Food Mart where Sterling was killed in the car park.

Alton Sterling is detained by two Baton Rouge police officers outside a convenience store. Moments later, one of the officers shot and killed Sterling. Photo: Screengrab/AP

"I'm heartbroken. It's outrageous. It's crazy," said Muflahi, who considered Sterling a friend and allowed him to sell CDs outside his store. He provided a copy of the video to Reuters and said police took a gun from Sterling's pocket.

Several hundred people gathered for a prayer vigil near the spot where Sterling was killed, with speakers urging peaceful protests, justice and unity in the face of "excessive force" used by police against black residents.

"If we stand divided, we are already defeated," Bishop Gregory Cooper, of Baton Rouge, told the crowd, including families with children, that filled the parking lot and nearby streets. Police stayed on the fringes of the gathering.

Stephanie McDee, a local blues singer, sings in protest at the shooting of Alton Sterling, at a makeshift memorial for him. Photo: AP

Many people stayed hours after the service and a brass band joined a large, peaceful nighttime march circling the store. A local artist had painted a mural of Sterling on the wall just feet from where he was shot.

Officials scrambled to defuse tension, saying there would be an independent investigation, after media showed a separate graphic video of the shooting recorded by a bystander.

"I have very serious concerns," Governor John Bel Edwards told reporters. "The video is disturbing, to say the least."

Protesters gather at City Hall in Baton Rouge over the shooting of Alton Sterling on Wednesday. Photo: Advocate/AP

Baton Rouge Mayor-President Kip Holden and police said they welcomed the probe launched by the Justice Department, the Federal Bureau of Investigation and federal prosecutors.

Such incidents undermine trust between police and the communities they serve, said presumptive Democratic presidential nominee Hillary Clinton.

"Something is profoundly wrong when so many Americans have reason to believe that our country doesn't consider them as precious as others because of the colour of their skin," she said in a statement.

Protesters gather near the Triple S Food Mart after Alton Sterling was shot and killed by Baton Rouge police. Photo: Advocate/AP

There was no immediate statement from presumptive Republican nominee Donald Trump.

Video recorded on the bystander's cell phone shows an officer confronting Sterling and ordering him to the ground. The two officers then tackle him to the pavement, with one pulling a gun from his holster and pointing it at his chest.

Muflahi's video shows the officers on top of Sterling. One of them yells, "He's got a gun." The video jerks away from the scene after the first two shots are fired.

Baton Rouge police chief Carl Dabadie Jr faces the media after the shooting. Photo: AP

Three more shots are heard, before the camera shows one officer lean over Sterling and take something from his pocket.

Police did not say how many shots were fired and declined to say whether a stun gun was used on Sterling. Muflahi said police tasered Sterling before he was tackled.

The two police officers involved, Blane Salamoni​, a four-year veteran, and Howie Lake, a three-year veteran, were both put on administrative leave, Baton Rouge police chief Carl Dabadie​ told a news conference.

People gather in protest outside of the Triple S Food Mart. Photo: Advocate/AP

The officers were responding to a call about a black man reported to have made threats with a gun, Dabadie said.

"When officers arrived, Sterling was armed and the altercation ensued that resulted in the loss of his life," he said.

The body cameras of both officers became dislodged during the altercation, but continued recording audio and video, police said. Those and other recordings will be turned over to federal investigators, police spokesman Lieutenant Jonny Dunnam​ said.

Family and friends of Alton Sterling protest on Tuesday afternoon. Photo: Advocate/AP

Court records show Sterling had several criminal convictions since the mid-1990s, for battery, resisting arrest, burglary and other crimes. He was a registered sex offender after spending close to four years in prison for felony carnal knowledge of a juvenile.

Relatives and acquaintances described Sterling as jovial and friendly, a neighborhood fixture who had peddled copied CDs, DVDs and games in front of the Triple S Food Mart for years.

Cameron Sterling, Alton's 15-year-old son, broke down crying at a news conference as his mother spoke. "He was killed unjustly and without regard for the lives he helped raise," said the mother, who did not give her name.

David Solomon, left, and Calvin Wilson, say they lived with Alton Sterling at the Living Waters Outreach Ministry Drop-in Centre, and don't believe he carried a gun. Photo: Advocate/AP

Several people were arrested for blocking traffic in downtown Philadelphia during a protest over the killing, broadcaster NBC Philadelphia reported.

The US Justice Department opened a civil rights investigation into the videotaped police killing of Sterling. Photo: AP

Children hold signs reading outside the store where Alton Sterling was shot and killed. Photo: AP

Reuters

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