NEW YORK Mayor Bill de Blasio received some boos and heckles yesterday at a police graduation ceremony, the latest chapter in his tension-filled relationship with the nation's largest police force.

The rift between de Blasio and much of the rank-and-file has grown considerably in recent weeks, and the leaders of the police union have blamed the mayor for fostering an anti-NYPD atmosphere they believe contributed to the ambush slayings of two officers earlier this month.

Twice in a week - including at the funeral for one of the officers - some officers turned their backs to de Blasio, adding an air of acrimony to the normally celebratory graduation ceremonies, which were held at Madison Square Garden.

The 884 new police officers sat stoically in their seats when de Blasio was introduced to speak and many in the audience tepidly cheered. But boos could be heard from some in the crowd in the seats reserved for cadets' families and friends.

About a dozen or so people in the stands stood with their backs turned to de Blasio, emulating the searing pose of disrespect that hundreds of officers struck at Officer Rafael Ramos' funeral on Saturday. Some appeared to be in uniform but it was unclear if they were members of the New York Police Department.

De Blasio, a Democrat elected last year on the promises of keeping crime low while reforming the NYPD, effusively praised the new officers.

Earlier yesterday, de Blasio oversaw a swearing-in ceremony for probationary firefighters. There were no overt signs of protest.