Dozens of pot-smoking squatters had their 4/20 mellows harshed when they were busted inside an empty Midtown commercial building on Monday, police said.

A concerned neighbor flagged down cops at 4:20 p.m. of all times after spotting five men head into an event space at 20 W. 23rd St., according to police.

When they walked in, the officers found 38 people in a smoke-filled room that reeked of marijuana on the third floor and issued each a summons, according to the NYPD.

The group was believed to be part of a larger party, but only the more-than three-dozen party-goers were caught green-handed, cops said.

The pot celebration was hosted by the “Ganja Pigs,” a company that specializes in edibles, according to NBC.

Cops found a duffel bag of marijuana, various edibles and THC, according to the report.

The pot party-goers were cited for unlawful trespassing, the report says. Five of them were given summonses for the drugs — but no one received one of the up-to $1,000 fines Mayor de Blasio has been promising for scofflaws since March, police said.