Those Chi-town ragers -- one with 1,000 guests, and another with a stripper getting swarmed -- were so packed, social distancing was impossible. Then again, face masks were rare at both events, so the guests clearly didn't care about COVID-19.

Cops, on the other hand, are very concerned about slowing the spread of the virus.

Our law enforcement sources tell us cops in big cities across the U.S. are NOT putting up with the dangerous practice, and they're starting to crack down more ... especially knowing there will be huge parties as it gets warmer.

We're told the Chicago, Los Angeles and New York Police Departments -- not to mention the L.A. Sheriff's Department -- are putting their ears to the ground and their eyes on social media to track down any emerging sardine-like get-togethers or turn-ups. That's right ... 5-0 is actively monitoring live feeds and trends, so if it starts to get poppin' and gets posted online -- like that stripper party was -- they'll probably see it.

We're told if the parties get too big and they're clearly violating basic quarantine guidelines, guests better not be surprised when cops come knocking to break it up. They might even start dinging people with citations too, depending on the circumstances.

Just for reference ... NYPD's dished out 190 summons and made some 22 arrests thus far pertaining to this exact behavior; LASD has cited 41 and arrested 3, and Chi-Town's at 13 citations and 18 arrests over flagrant breaking of social distancing. More to come, we're sure.