A Riverhead man was arrested and charged with first-degree assault after police said he stabbed and critically injured another man during a dispute on West Main Street in Riverhead late Monday night — a dispute witnessed by a plainclothes Riverhead Town police officer, who gave chase and helped make the arrest.

Police said the victim, identified only as a 36-year-old man, collapsed on the street while trying to flee the assailant. He was taken by a Riverhead Ambulance crew to nearby Peconic Bay Medical Center in critical condition with life-threatening injuries, police said.

The victim had been stabbed multiple times, police said, and, following emergency surgery to stabilize his injuries, he remained in critical condition in intensive care.

The assault unfolded on West Main Street near Griffing Avenue when, police said, the plainclothes officer "observed some kind of dispute" between the men at about 10:45 p.m.

As the officer called in the disturbance on his radio, police said he saw the suspect begin chasing the victim, who "appeared to be bleeding, his shirt soaked in blood."

Police said the officer saw the victim cross West Main and collapse on the street in front of a row of storefronts. The suspect who had been chasing him then ran west on West Main, as the plainclothes officer gave chase, police said.

The suspect, identified as Ronald Dejesus Oscal Cruz, 29, was arrested without incident a short distance away. A large kitchen knife "covered in blood" was in his possession when arrested, police said.

In addition to the assault charge, police said Cruz was charged with fourth-degree criminal possession of a weapon. The investigation into the incident ongoing.

Police did not detail the events leading to the assault.

Arraignment information was not immediately available Tuesday.

Police are asking anyone with information regarding the incident to call detectives at (631) 727-4500.

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