Hundreds of protesters have rallied for a second day of demonstrations in the US city of Pittsburgh to demand justice for an unarmed African American teenager who was fatally shot by police officers as he was running away.

The protesters first took to the street on Wednesday evening, and fresh rallies erupted on Thursday in front of the Allegheny County court house.

More than 200 activists gathered for Thursday’s rally, chanting “black lives matter” and “justice for Antwon.”

Antwon Rose, 17, was shot three times by a police officer near Pittsburgh as he ran from the vehicle on Tuesday. A video of the incident was recorded by a bystander’s mobile phone and posted on social media.

The video shows Rose and another man getting out of the passenger side of a car and running when multiple shots are heard immediately after.

Police have indicated that they believe the car was connected to a drive-by shooting they had been investigating.

The shooting officer, whose name has not been released, had been sworn into the police department just a few hours prior to the incident.

Allegheny County Police Superintendent Coleman McDonough said Rose was unarmed but two semi-automatic handguns were found on the floor of the vehicle. He said the driver of the vehicle was arrested and released without charges.

Antwon was a “generous, hard working and highly promising student”, his family said in a statement, which was provided by civil rights activist and attorney S Lee Merritt, who is representing the family. “These facts, without more, simply leave very little room to justify these of deadly force by this officer.”

Pittsburgh Mayor William Peduto said this is “a devastating situation” and he is saddened for Rose and his family.

Fatal police shootings and other forms of violence against African Americans by police have sparked massive protests across the US in recent years.

US police killings of blacks exact mental health toll

The disproportionately high rate at which unarmed black people die at the hands of police in the United States has a corrosive impact on the mental health of black Americans, researchers reported Friday.

The frequency of these killings has been cited as symptomatic of deeply rooted racism, and is in any case perceived as such by most black Americans, they reported in The Lancet, a medical journal.

“We found that when police kill unarmed black Americans, there is mental health fallout that reverberates throughout the black American community,” said senior author Alexander Tsai, an associate professor of psychiatry at Massachusetts General Hospital and Harvard Medical School.

“This finding is significant because it shows that the effects of these killings go beyond immediate friends and family,” he told AFP.

Tallies kept by news organizations and researchers vary, but police have killed approximately 300 black Americans – about a quarter of them unarmed – each year since 2014.