By Sebastien Malo and Katie Reilly

NEW YORK (Reuters) - A New York man accused of shooting a plainclothes police officer last month pleaded not guilty to murder charges on Thursday.

Demetrius Blackwell, 35, was arrested hours after he allegedly shot New York police officer Brian Moore, 25, in the head after Moore attempted to question him in a middle-class section of the borough of Queens on May 2. Moore died on May 4.

Blackwell was also charged with the attempted murder of Moore's partner, Erik Jansen. He pleaded not guilty to all 12 criminal counts he faces, including aggravated assault on a police officer, first-degree assault and weapons possession.

If convicted of aggravated murder, he faces a mandatory sentence of life in prison without the possibility of parole.

Blackwell, handcuffed and wearing a bright orange prison uniform, spoke only one word during his brief court appearance, saying "yes" when asked to identify himself.

Moore's father kept his eyes on Blackwell throughout the proceedings, in a courtroom packed with police officers, some in uniform and others wearing blue T-shirts that read "In memory of P.O. Brian Moore - End of watch 5/04/15."

Blackwell's attorney, David Bart, said he may pursue a defense based on mental illness.

Earlier, Queens County District Attorney Richard Brown ruled out any plea deal, saying "the defendant's alleged conduct, and his total disregard of the law and authority, clearly demonstrate that he is a serious threat to society."

Moore's death came at a time of fraught relations between police and minority communities across the United States.

Police killings of unarmed black men in New York, Missouri, South Carolina and elsewhere have prompted months of largely peaceful protests punctuated with bouts of arson and looting.

A gunman who in December shot and killed two uniformed New York police officers as they sat in their patrol car in Brooklyn left postings on social media suggesting his attack was an act of revenge for police killings of black men.

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Blackwell is black, and Moore was white.

Moore's shooting did not appear to be politically motivated and his funeral last month featured none of the ire seen at ceremonies for the officers killed in Brooklyn, where thousands of New York police officers in uniform turned their backs on Mayor Bill de Blasio, whom police union officials had accused of not adequately supporting officers.





(Editing by Scott Malone, Doina Chiacu and Eric Beech)