At least 11 people were shot dead and around 70 others wounded in 48 hours of violence in Chicago at the weekend.
Police Patrol Chief Fred Waller said gang members are using large summer crowds as cover in some cases.
Waller added: "They take advantage of that opportunity and they shoot into a crowd, no matter who they hit."
Police said violent crime has declined overall in Chicago. But anti-violence protesters have blocked highways to voice their outrage.
The violence peaked early on Sunday, including one shooting on the city’s South Side that wounded eight people.
Police attributed the spike in violence to gangs and US President Donald Trump’s lawyer, Rudy Giuliani used it to criticise the city’s Democratic leadership.
By comparison, at least seven people were killed and 32 were wounded during the long Memorial Day weekend in May, which is one of the most violent weekends of the year for the city, the Chicago Tribune reported.
Chicago ended 2017 with fewer homicides, 650, than 2016, when there were 771. Although the drop was significant, it exceeded the combined number of killings in New York City and Los Angeles, which are the two US cities bigger than Chicago.
Chicago’s gun violence has drawn widespread attention, including from Mr Trump and his lawyer, Mr Giuliani, who blamed the problem on Chicago’s long-time "Democratic rule" in a series of tweets on Sunday and Monday.