Hundreds of protesters stood vigil early on Thursday outside a convenience store in Baton Rouge, Louisiana, demanding the prosecution of police who fatally shot a black man there two days earlier.

The demonstration was largely peaceful at about 1 a.m. local time as about 300 protesters remained outside of the Triple S Food Mart, where Alton Sterling, 37, was pinned to the ground and fatally shot in the chest by two white police officers on Tuesday.

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"There is not going to be a riot until they show they are not going to prosecute these people," said Arsby, a 53-year-old truck driver who declined to give his last name, as he stood outside of the store. "Right now it's just started."

Some protesters blocked traffic while others marched, sang, and chanted, accusing the police of using "excessive force" against black residents.

"If we stand divided, we are already defeated," Bishop Gregory Cooper of Baton Rouge told the crowd. Police stayed on the fringes of the gathering.

Graphic video images of Tuesday's shooting of Alton Sterling, 37, unleashed protests and social media outcry over 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 pants pocket as he was dying, according to images recorded by Abdullah Muflahi, owner of the store where Sterling was killed in the parking lot.

The U.S. Justice Department said on Wednesday it would investigate the killing. 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.

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"Now, all eyes are in Baton Rouge. What may have been easier to cover up before because it was just us ... now they've woken up the sleeping lions," said protester Tammara Crawford, a 33-year-old mother and school administrator in Baton Rouge.

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.

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

The two police officers, Blane Salamoni and Howie Lake, have been put on administrative leave, police said.

Court records show Sterling had several criminal convictions and he was a registered sex offender after spending close to four years in prison for felony carnal knowledge of a juvenile.

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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.