New York City detectives are investigating allegations that a sexual assault was committed last week in the Upper East Side apartment of a prominent lawyer with a long history of pursuing high-profile lawsuits over police misconduct, the police said.

A 43-year-old woman said that she was raped early Thursday by the lawyer, Sanford A. Rubenstein, according to a law enforcement official, who spoke on condition of anonymity because no charges had been filed.

Detectives from the Manhattan Special Victims Squad have been investigating the complaint since Friday evening, when the woman contacted the police.

A message left for Mr. Rubenstein on his cellphone on Sunday was not immediately returned. Lawyers representing him, Benjamin Brafman and Michael S. Ross, said in a statement Sunday that Mr. Rubenstein “denies any criminal conduct whatsoever” and that they are “confident that when the investigation is completed, it will be closed without the filing of any criminal charges.” Reached by phone, Mr. Ross declined to answer questions about the matter.