NEW YORK — Police officers from around the U.S. joined grieving family members as they mourned a 32-year-old immigrant who realized his dream of being an NYPD officer but was gunned down in his squad car.

New York City Mayor Bill de Blasio and Police Commissioner William Bratton eulogized Officer Wenjian Liu on Sunday as an incarnation of the American dream: a man who had emigrated from China at age 12 and devoted himself to helping others in his adopted country.

But thousands of officers turned their backs as de Blasio spoke, repeating a stinging display of scorn for a mayor who many officers said is anti-police.

The gesture among officers watching the mayor’s speech on a screen outside a Brooklyn funeral home added to tensions between de Blasio and rank-and-file police even as he sought to mollify them.

“Let us move forward by strengthening the bonds that unite us, and let us work together to attain peace,” de Blasio said.