A gunman shot someone and then barricaded himself inside a Greenpoint building Monday afternoon, prompting NYPD snipers to perch on nearby rooftops and other officers to flood the area, authorities said.

The Brooklyn man was eventually taken into custody,officials said.

“Holy s–t, someone just got shot on #Greenpoint Ave,” tweeted Camille Raneem, who works at a shop nearby. “Shooter locked down inside 183.”

At least one victim was shot on Greenpoint Avenue near McGuinness Boulevard at around 2:30 p.m. and rushed to Bellevue Hospital in critical condition, according to police.

The shooter holed up in 183 Greenpoint Ave. after fleeing inside moments after the incident, police sources said.

Numerous cops were spotted outside the building — with officers and snipers mobilized on rooftops — around 3:30 p.m., before finally apprehending the suspect.