Pallbearers carry the casket of slain New York Police Department officer Wenjian Liu in the Brooklyn borough of New York January 4, 2015. [Photo/Agencies]

During his 2013 campaign for office, the mayor had criticized some of the NYPD's tactics, including a "stop-and-frisk" policy that critics said was used to harass African-Americans and other minority groups.

The mayor also offered qualified support for the wave of protests triggered by the two black men's deaths in New York and Ferguson, Missouri. He said he had talked to his bi-racial son, Dante, about being wary in dealing with police.

In a sign of the police force's broadening ethnic diversity, observances for Liu are expected to meld Chinese and Buddhist customs with the usual traditions of an NYPD funeral, which date to a time when Roman Catholic men of Irish or Italian descent dominated the force.

Caiyao Chen, 32, was among a crowd of mostly Asian mourners who pressed against barricades on the sidewalk outside the services. Across from them, police officers with white gloves and dress blues stood guard in silence as they awaited for the funeral to begin.

Chen, who moved to the United States from China in 2000, didn't know Liu, but he said he was particularly saddened because his death as the only son of a Chinese family irrevocably broke their lineage.

"In Chinese tradition the son carries the blood of the family," he said. "The family is broken now."