Officials may go so far as to shut down establishments that are violating social distancing rules, de Blasio said, asking residents to call 311, the city's non-emergency hotline, if they see such crowding.
Bars and restaurants in the city have been restricted to takeout and delivery service since mid-March, when coronavirus cases started to soar, but some in Manhattan were allowing people to dine and drink inside on Saturday.
"We're not going to tolerate people starting to congregate. It's as simple as that," de Blasio said. "If we have to shut places down, we will."
After a rash of violent social distancing arrests involving people of color, the city eased up on social distancing enforcement this week by no longer having police officers breaking up small groups of people or confronting citizens about failing to wear a mask.
As the weather heats up, though, more and more New Yorkers are flocking to public spaces and familiar haunts for a sense of normalcy after spending most of the last two months cooped up inside - and not always policing themselves.
"If you start to form groups of people and then two, three, five and then it becomes six, it becomes 10, it becomes 15 - that violates what we're saying about social distancing and that puts lives in danger," de Blasio said.
Parks, boardwalks and beaches attracted big crowds on Saturday, though city beaches aren't officially open and won't be for the upcoming Memorial Day weekend.
Beaches on Long Island and in other parts of the region will be open for the holiday, but de Blasio said opening the city's grand strands for swimming and merriment "is not safe" and "is not the right thing to do in the epicenter of this crisis."
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