Dozens of protesters in Louisiana on Tuesday chanted slogans and held up signs demanding justice for a black man fatally shot in an altercation with two police officers hours earlier, video postings on social media showed.

The shooting comes at a time of fierce national debate and heightened scrutiny over the use of excessive force by police, especially against black men, in major U.S. cities, such as New York, Baltimore and Chicago.

Posts on Twitter showed the demonstrators gathered outside the Triple S Food Mart convenience store in Baton Rouge, where, police said in a statement, the man, Alton Sterling, 37, was shot by officers soon after midnight.

"No justice, no peace," chanted the protesters, who held up signs and occasionally blocked traffic, in images transmitted by media outlets in Baton Rouge.

Police officials were not immediately available to comment on the shooting or the protest. Reuters could not immediately trace relatives of Sterling, or a representative, to seek comment.

At about 12:30 a.m. on Tuesday, two police officers responded to a disturbance at the convenience store where they encountered Sterling, the Baton Rouge Police Department said.

"Uniformed officers responded to a disturbance call from a complainant who stated that a black male who was selling music CDs and wearing a red shirt threatened him with a gun," it added.

Sterling was shot in the ensuing altercation and died at the scene. The officers where placed on administrative leave, police said.

Several news stations on Tuesday evening aired what they said was cell phone video of the incident. It showed an officer using a stun gun on a red-shirted black man in a store parking lot and ordering him to get on the ground.

The two officers then tackled the man to the ground, and one pulled a gun from his holster to point it at the man's chest, the video showed.

At least three gunshots then ring out on the video clip, followed by the sound of a woman screaming and crying and a man asking whether "they shot him."