NEW YORK (REUTERS) - New York City police guards saluted as Mayor Bill de Blasio entered a Brooklyn funeral home on Saturday with his police commissioner for the wake of the second of two patrol officers killed in an ambush last month.

Ahead of the wake for Wenjian Liu, believed to be the city's first Chinese-American officer killed in the line of duty, Commissioner Bill Bratton told his force to refrain from the "act of disrespect" seen at the funeral of Liu's partner, when some of those in uniform turned their backs on Mr de Blasio.

"A hero's funeral is about grieving, not grievance," Bratton wrote in a memo to be read on police roll calls over the weekend.

Liu, 32, and Rafael Ramos, 40, were shot to death on Dec 20 as they sat in their squad car in Brooklyn.

Their killer, Ismaaiyl Brinsley, who killed himself soon after, had said he was seeking to avenge the deaths this summer of two unarmed black men at the hands of white police officers.

The killing of Liu and Ramos has strained the already frayed relations between the rank and file and Mr de Blasio, who was a critic of police policies when he ran for office in 2013.

The mayor, who has a biracial son, has also offered qualified support for the wave of protests triggered late last year by the black men's deaths in New York and Missouri.

Immediately after Liu and Ramos were shot, Patrick Lynch, the head of the city's largest police union, expressed scorn for Mr de Blasio.

"There is blood on many hands," he said. "It starts on the steps of City Hall, in the office of the mayor."

On Saturday, a few hundred mourners, a majority of whom were police officers in dress blue uniforms, queued on a frigid and snowy afternoon outside the funeral home.

Mr De Blasio and Mr Bratton entered together the service together shortly after the funeral home opened, with officers standing guard by the funeral's entrance saluting both men as they made their way inside.

The funeral for Ramos last Sunday was among the largest in the history of the department, with more than 20,000 officers from around the country filling streets around the church.

When Mr de Blasio began his eulogy there, many uniformed officers turned their backs on television monitors set up outside, in a gesture of disdain for the liberal mayor following his criticisms of police policies.

"For the last seven days, the city's and the country's consciousness has focused on an act of disrespect," said Mr Bratton, who had previously called the action inappropriate.

He said it had stolen the "valor, honour and attention" that rightfully belonged to the slain officer.

BROADENING DIVERSITY

Services for Liu were taking place in Bensonhurst, Brooklyn, a mixed neighborhood with a large Chinese population where Liu lived with his wife of two months and his parents.

A wake, closed to the public, was to be followed by a funeral that tens of thousands of police were expected to attend.

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

Waiting by herself outside the wake, Winnie Liu, 62, held a bouquet of white flowers traditionally brought to Chinese funerals.

She said she felt a kinship with Liu because she shared his last name and both their mothers' surname was also the same, Lee.

"From the first day I felt that I was from the same family so I'm taking this very, very hard," she said. "All the Chinese I know are upset; we feel together in pain, supportive of the family and the NYPD."

Frank, a retired NYPD detective who declined to give his last name, said he had attended funerals of all police officers who fell in the line of duty. His partner had died in his arms while on a patrol in 1971, he said.

"We just do it out of respect," he said. "Cops are brothers."

In his memo, Mr Bratton said he understood emotions were running high among the rank and file after what he described as the assassination of the two officers.

He said his entreaty to the department was not a mandate and he was not threatening to discipline those who did not comply.

"But I remind you that when you don the uniform of this department, you are bound by the tradition, honour and decency that go with it."