Two young adults died after the shooting in Fairmount Park, police Commissioner Kevin Bethel said.
Those injured in the shooting were listed in stable condition, he said.
Police said no arrests have been made.
"This is a heinous act of violence that was inhumane. No regard for life," Mayor Cherelle Parker said at a news conference.
"We will not be held hostage by anyone who decides that they want to get assault-like, warlike weaponry."
It wasn't yet clear what led up to the shooting or whether those who were shot were targeted, the police commissioner said.
No weapons had been recovered, but investigators found casings from three weapons, Bethel said.
From the rapid-fire gunshots heard in videos shared on social media, he said they can determine that a switch, which automates a semiautomatic weapon, was used.
"That's the sound of war. When you have an automatic weapon that you can empty a magazine - a 20-clip magazine - in seconds," Bethel said. "It is meant to kill, to create carnage and to hit as many people as possible. And in this case, you see, it was able to do that."
Police identified those killed as Amya Devlin, 23, and Mikhail Bowers, 21. The wounded include a 15-year-old boy and two girls, ages 16 and 17, police said.
he other injured people, four women and two men, range in age from 18 to 28.
Bethel did not know how many people were at the park at the time of the shooting, but noted that estimates were in the hundreds. The shooting occurred despite officers' presence in the park, Bethel said.
"We try to manage the crowds as they unfold, but at the same token, it is a challenge when individuals decide that they're going to fire into a crowd," Bethel said.
Officers were called in to disperse crowds earlier in the evening and met with adults, who agreed to start shutting down, but Bethel said it took time and there a large number of young people blended into the crowd.
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