The violence erupted as many, including US president Joe Biden, call on Congress to enact meaningful gun control measures, especially in the wake of deadly mass shootings last month in Buffalo, New York, and Uvalde, Texas.
In Philadelphia, police officers were patrolling the downtown area on South Street when numerous gunshots rang out, and they witnessed several suspects firing into a large crowd just before midnight, city police inspector D F Pace said during a news conference.
An officer fired at one of the suspects from about 30 feet away, Pace said, but it was unclear if that person was struck.
The conditions of those who were wounded by gunfire remained unknown on Sunday afternoon.
Investigators processing the scene recovered two handguns, including one with an extended magazine, police said. No arrests were immediately announced. Pace said police were asking business owners to review video from surveillance cameras.
The department said on Twitter late Saturday that people should avoid the area, which is home to night life entertainment venues, bars, restaurants and others businesses.
Meanwhile, the shooting in Tennessee occurred near a nightclub in Chattanooga about 2.40am. Three were killed, 14 were struck and injured by bullets, and three were hit by cars while trying to flee the chaotic scene on McCallie Avenue, police there said.
The Chattanooga police chief, Celeste Murphy, suggested investigators suspected there were multiple shooters drawn into a dispute among each other, but detectives were still trying to figure out what sparked the gunfire.
Sixteen of the wounded in Chattanooga were adults, and one was a minor, police said. Six other minors were injured in a shootout the previous weekend in Chattanooga.
