Before the trouble with his current girlfriend at the shelter, the two of them would save money from panhandling and go on long drug benders across the city, followed by slow recoveries in shelters and on the streets.

A smile suddenly flashed across Mr. Morgan’s face, exposing the gaps where his front teeth had been. His girlfriend had arrived. She pulled out an envelope filled to the brim with the dry green herbs of K2. She measured out half of it for him, spilling a bit on the sidewalk.

Mr. Morgan rolled a joint and lit up.

“If I weren’t on this I might be angry, I might be hitting her. Who knows what I would be up to,” he said, his words trailing off with each drag of the cigarette. “This stuff makes me calm.”

After a while he came to and looked up at his girlfriend.

“Do you want to stay with me here tonight? We can have some fun in the grass, then sleep under the bench.”

Image Mr. Morgan, who is homeless, stores some of his clothing in a pair of sweatpants with the legs tied to create a kind of storage bag. Credit... Josh Haner/The New York Times

Police raids on bodegas on this street in July ended in confiscations of more than 8,000 packets of K2, but many of the stores continue to sell the drug, those on the street say. That has left police officers looking on as drug users light up in plain sight. Then the officers haul them into ambulances as they drop onto the street.

The sheer number of users has left officers on edge on this block. “It quickly can become a kind of group mentality where the officers, or even multiple officers, are outnumbered,” Deputy Inspector Tom Harnisch, commander of the 25th Precinct, said.