Police officers from around the city and nation gathered Sunday morning to give their final salutes to slain NYPD Detective Wenjian Liu.

Funeral services for Liu began were held at the Ralph Aievoli & Son Funeral Home in Brooklyn's Dyker Heights neighborhood.

Liu and his partner, Detective Rafael Ramos, were killed in their patrol car during an ambush shooting on Dec. 20. The gunman took his own life in a nearby subway station.

The two officers were promoted to the rank of detective posthumously.

Liu's funeral included remarks from his father, cousins and his widow, who tearfully spoke about how her husband of just two months was a respectful and hardworking man who did much for the Chinese community.

As Mayor Bill de Blasio gave solemn eulogy at Liu's funeral, thousands of police officers watching the service from outside again turned their backs in a sign of disrespect toward the city's leader.

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

The final tribute also included a Chinese ceremony led by Buddhist monks and a traditional police ceremony.

Detective Liu's wake was held Saturday at the same funeral home as today's service. It was attended by Gov. Andrew Cuomo, just two days after the death of his own father, former Gov. Mario Cuomo.

Mayor de Blasio and Commissioner Bratton also made appearances at the wake, and they were saluted by officers as they entered the funeral home.

Following the service, Liu was buried at Cypress Hills Cemetery, where his partner was also laid to rest.

Liu was a seven-year veteran of the force.