New York Mayor Bill de Blasio has given a solemn eulogy at a funeral for a slain officer as thousands of police watching the service from outside turned their backs in a sign of disrespect toward the city's leader.

The officers' actions at Wenjian Liu's funeral today came after Police Commissioner William Bratton sent a memo to all commands urging respect, declaring "a hero's funeral is about grieving, not grievance".

Liu was killed along with partner Officer Rafael Ramos on December 20. Their killer committed suicide.

The deaths strained an already tense relationship between city police unions and the mayor.

Last weekend, hundreds of officers outside Ramos' funeral turned their backs toward a giant TV screen as de Blasio's remarks were being broadcast.

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