A post-mortem examination showed the victim, Alton Sterling, 37, died of gunshot wounds to the chest and back.
Protesters later blocked nearby roads.
The crowd of about 200 people was moved on by police but organisers say they would reassemble in front of City Hall later.
The incident comes amid heightened tension in the US over the deaths of African-American men at the hands of police.
There are more than 1,000 deadly shootings by police in the US each year, and those killed are disproportionately black Americans.
Police spokesman Cpl L`Jean McKneely said the officers had been called in after reports that a man selling CDs outside a convenience store had threatened someone with a gun.
Mr Sterling, a father of five, died at the scene.
Cpl McKneely said that the two officers involved had been placed on administrative leave, which is standard procedure.
Reporter Brittany Weiss from local TV station WBRZ told the BBC`s Newsday programme that the video footage, which emerged hours after the shooting, showed the officers having an altercation with the man in a car park.
"The police officers bring this man down to the ground and then you hear more of an altercation," she said.
"Then you hear three gunshots and two other shots that are muffled and then you hear someone say `he has a gun` during that exchange."
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