A 40-year-old man died from a blow to the head during a clash inside a Harlem apartment building early Saturday, police said.

Cops called to the Frederick Douglass Blvd. building near W. 120th St. about 3:15 a.m. found the victim unconscious suffering from a massive head injury, police said.

EMS rushed him to Mount Sinai Morningside Hospital, but he couldn’t be saved. His name was not immediately disclosed as cops track down relatives.

An autopsy has been slated to determine how the man died.

Another man, identified by police as a “person of interest” was taken into custody at the scene and was being questioned Saturday.

No charges were immediately filed.