Chicago police plan a crackdown on unsanctioned street and block parties after police found that many of the victims of last weekend’s violence had attended such gatherings.

At a press conference Thursday at police headquarters, Supt. Eddie Johnson said that an examination of the violence this past weekend when 71 people were shot, including 12 fatally, found that 20 percent of victims had attended street parties that hadn’t been officially permitted by the city. While block parties in good weather are not out of the ordinary in Chicago, Johnson said there was an unusually large number of such unauthorized gatherings last weekend.

To combat gun violence the rest of the summer, the department plans to implement “emergency hotspot dispersal zones” in 30 gang-linked areas in the 5th, 6th, 10th, 11th and 15th police districts. Those were the five districts where most of last weekend’s shootings took place.

Officers within these zones will take extra measures to police any unsanctioned large gatherings of 10 or more people. People attending the parties will be first asked to leave. If they refuse, they face arrest, Johnson said. The heightened enforcement will continue for at least a month, police said.

Social media has made it easier for the gatherings to grow rapidly and made policing them more difficult.

“Years ago, a large gathering really had to be advertised for it to become a large gathering,” he said. “Now we’re having people livestream from a particular location — ‘We’re going to be having a party’ — and 10 people will turn into 200 just like that.”

Many of these unsanctioned parties were block parties, likely with a gang connection, Johnson said. Rival gang members showed up, Johnson said, and likely fired into the crowds at random.

Johnson emphasized that police aren’t targeting block parties specifically, and said cops will evaluate a number of criteria before breaking up a gathering. Neighborhood block clubs will be contacted to ensure they know to register parties with the city and to offer CPD as a safety resource.

“There’s nothing wrong with people enjoying nice weather in this city,” Johnson said. “The goal isn’t to be arresting people just because they’re enjoying summertime.”