Thousands of police and other mourners were expected to fill a New York City church and surrounding streets for the funeral Saturday of one of two police officers ambushed by a gunman who said he was avenging the killing of unarmed black men by police.

U.S. Vice President Joe Biden was due to make remarks at the service for Rafael Ramos, whose death, alongside his partner Wenjian Liu has become a rallying point for police and their supporters around the country, beleaguered by months of street rallies by protesters who say police practices are marked by racism.

"This is probably going to be the largest one, maybe in the history of the police department," Stephen Davis, the department's chief spokesman, said of the funeral, which is drawing delegations from police forces across the United States.