Police say the 33-year-old victim was assaulted by two men on Richmond Terrace around 2:35 a.m. Thursday.
The suspects fled the scene in a taxi, leaving the victim lying in the street in front of the ferry terminal.
He was then accidentally run over by a passing black Ford Explorer driven by the off-duty cop. The driver, who did not see the victim lying in the street, stopped and called police.
The victim was rushed to Richmond University Medical Center in serious but stable condition.
The driver of the SUV was talking to investigators.
The two suspects in the initial assault have not been caught.
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