An off-duty Nassau County paramedic who had come to the aid of a man he believed to be under the influence of "an unknown substance" was instead attacked by the man Wednesday evening in Syosset, Nassau County police said.

The paramedic, an advanced medical technician whose identity was not released by police, suffered lacerations and contusions that required treatment at a hospital, where he also received stitches to his hand.

His attacker was wrestled to the ground by a passerby -- and was later arrested.

Randolfo A. Catu, 43, of 89-31 205th St., Hollis, Queens, faces second-degree assault and second-degree reckless endangerment charges after his arrest. He is scheduled to be arraigned Thursday in First District Court in Hempstead.

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Police said that Catu was observed walking on Cold Spring Road, stumbling into the road, by the off-duty paramedic at about 7:30 p.m. The paramedic, identified only as a 56-year-old man, called 911 for assistance and, police said, attempted to assist Catu.

Instead, police said, Catu "became irate," pushing the paramedic into the road and "punching the victim several times," causing the injuries.

A 39-year-old man who witnessed the assault then came to the aid of the paramedic, holding Catu for arrest, police said.

The suspect also was taken to a hospital for evaluation, police said.