New York (CNN) The ceremonial burning of paper money and the melodic sounds of Buddhist chants are marking a weekend of remembrance for the second police officer killed in an ambush by a lone gunman in December -- a sendoff that reflects the changing face of a department once known for its deep Irish and Italian roots.

Under a mix of rain and snow, Det. Wenjian Liu was remembered Saturday at Aievoli Funeral Home in Brooklyn, where Mayor Bill de Blasio, Police Commissioner William Bratton, local officials and throngs of uniformed police officers and firefighters gathered at his wake in a sign of the diversity and unity of the nation's largest city.

Liu was 12 when his parents emigrated with him in 1994, coming to the United States from Canton, China. His police partner, Rafael Ramos, a New York native of Puerto Rican descent, was remembered last week with a massive outpouring at a Protestant church in Queens.

With the city and nation divided over the treatment of minorities by their brothers and sisters on the force, the lives of the two fallen officers more closely reflected the city's diversity.

On Sunday, a sea of dark blue drawn from the police forces of New York and beyond is expected to find its way to the funeral home for Liu's funeral at 11 a.m. ET.

When his NYPD colleagues gather there, their boss has an urgent request of them: Please remain reverent throughout the services.

Don't carry out an "act of disrespect" against anyone, Police Commissioner William Bratton said in a memorandum obtained by CNN.

Turning their backs

The message was directed at a large group that attended the funeral for Ramos last week and turned their backs on de Blasio as he gave his eulogy. On Saturday, several officers saluted de Blasio when he arrived at the funeral home.

Photos: Honoring slain NYPD Officer Rafael Ramos Photos: Honoring slain NYPD Officer Rafael Ramos Honoring slain NYPD Officer Rafael Ramos – Law enforcement helicopters fly over Christ Tabernacle in Queens, where the casket of New York City police officer Rafael Ramos is carried by pallbearers, on Saturday, December 27. Ramos and Officer Wenjian Liu were shot and killed while they sat in their patrol car in Brooklyn on December 20. Hide Caption 1 of 19 Photos: Honoring slain NYPD Officer Rafael Ramos Honoring slain NYPD Officer Rafael Ramos – The casket of Officer Ramos is carried out of Christ Tabernacle. Hide Caption 2 of 19 Photos: Honoring slain NYPD Officer Rafael Ramos Honoring slain NYPD Officer Rafael Ramos – Maritza Ramos, center, wife of Officer Ramos, holds the colors as she is flanked by her sons Justin, left, and Jaden during the funeral service. Hide Caption 3 of 19 Photos: Honoring slain NYPD Officer Rafael Ramos Honoring slain NYPD Officer Rafael Ramos – Police officers hold hands in prayer during the funeral services. Hide Caption 4 of 19 Photos: Honoring slain NYPD Officer Rafael Ramos Honoring slain NYPD Officer Rafael Ramos – Justin Ramos is comforted by his mother following the funeral service. Hide Caption 5 of 19 Photos: Honoring slain NYPD Officer Rafael Ramos Honoring slain NYPD Officer Rafael Ramos – Police officers salute as the casket of Officer Rafael Ramos arrives before the funeral. Hide Caption 6 of 19 Photos: Honoring slain NYPD Officer Rafael Ramos Honoring slain NYPD Officer Rafael Ramos – Thousands of police officers gathered to pay their respects outside the church. Hide Caption 7 of 19 Photos: Honoring slain NYPD Officer Rafael Ramos Honoring slain NYPD Officer Rafael Ramos – New York City Mayor Bill de Blasio and his wife, Chirlane McCray, arrive for the funeral. Hide Caption 8 of 19 Photos: Honoring slain NYPD Officer Rafael Ramos Honoring slain NYPD Officer Rafael Ramos – NYPD officers embrace before the funeral of the slain officer. Hide Caption 9 of 19 Photos: Honoring slain NYPD Officer Rafael Ramos Honoring slain NYPD Officer Rafael Ramos – Vice President Joe Biden and his wife, Jill, enter Christ Tabernacle Church in Queens for the funeral of the slain NYPD Officer. Hide Caption 10 of 19 Photos: Honoring slain NYPD Officer Rafael Ramos Honoring slain NYPD Officer Rafael Ramos – Police Commissioner Bill Bratton, center, arrives with his wife, Rikki Klieman. Hide Caption 11 of 19 Photos: Honoring slain NYPD Officer Rafael Ramos Honoring slain NYPD Officer Rafael Ramos – Officers from Ramos' precinct wait outside before the funeral. Hide Caption 12 of 19 Photos: Honoring slain NYPD Officer Rafael Ramos Honoring slain NYPD Officer Rafael Ramos – New York Gov. Andrew Cuomo arrives at the funeral with his girlfriend Sandra Lee. Hide Caption 13 of 19 Photos: Honoring slain NYPD Officer Rafael Ramos Honoring slain NYPD Officer Rafael Ramos – Justin Ramos arrives at the funeral wearing his father's jacket. Hide Caption 14 of 19 Photos: Honoring slain NYPD Officer Rafael Ramos Honoring slain NYPD Officer Rafael Ramos – Family members of officer Ramos, including sons Jaden and Justin and his wife, Maritza Ramos, in backseat, arrive for the funeral. Hide Caption 15 of 19 Photos: Honoring slain NYPD Officer Rafael Ramos Honoring slain NYPD Officer Rafael Ramos – Police officers gather outside the church. Hide Caption 16 of 19 Photos: Honoring slain NYPD Officer Rafael Ramos Honoring slain NYPD Officer Rafael Ramos – An NYPD officer loads flowers onto a vehicle outside Officer Ramos' funeral. Hide Caption 17 of 19 Photos: Honoring slain NYPD Officer Rafael Ramos Honoring slain NYPD Officer Rafael Ramos – Church staff place flowers on a vehicle before the funeral. Hide Caption 18 of 19 Photos: Honoring slain NYPD Officer Rafael Ramos Honoring slain NYPD Officer Rafael Ramos – Firefighters arrive for the funeral. Hide Caption 19 of 19

It was a repeat protest. They had done it before, the night after the two men were shot dead in their car, when de Blasio entered the hospital to condemn the killing.

Some officers accused the mayor of encouraging anti-police fervor due to his support of protests against police brutality. Tens of thousands had taken to the streets in the wake of the death of African-American man Eric Garner during an encounter with white officers.

Pat Lynch, head of the Patrolmen's Benevolent Association, has taken aim at de Blasio, even saying the mayor's office was stained with the officers' blood. He has called for searing protest by police officers, and some have heeded that call.

But for Sunday's funeral, Bratton is putting his foot down at a potential repeat by some officers.

'Grieving, not grievance'

"A hero's funeral is about grieving, not grievance," Bratton said. The last display put all NYPD officers in a bad light, he said.

"It stole the valor, honor and attention that rightfully belonged to the memory of detective Rafael Ramos's life and sacrifice," his memo read.

But he commiserated with the sentiments of many officers.

"As a cop, one who lived and worked through the assassination threats of the 1970s, I understand that emotions are high," he said.

Still the message was clear: Respect the fallen; honor of the uniform.

He expressly said there'd be no punishment for noncompliance at Liu's funeral but made sure the request was heard by all. His memo was to be read or displayed at police roll calls through Sunday.

The ambush

Ramos and Liu were sitting in their squad car in Brooklyn when Ismaaiyl Brinsley ambushed them with a gun December 20. He shot himself to death in a nearby subway station.

Brinsley had a lengthy rap sheet that included gun crimes. Before killing the two men, he had tweeted messages that he would kill police officers in the wake of the deaths of Garner and Michael Brown in Ferguson, Missouri.

Bratton promoted Liu and Ramos to the rank of detective first grade during Ramos' funeral, to rousing applause.

Wenjian Liu

Liu's family came to the United States from Canton, China, in search of a better life.

He majored in accounting in college, but he opted for a different path, joining the New York Police Department in 2007. He was proud to serve as an officer, his family said, using his Chinese-language skills whenever they were needed.

He got married in September and had been looking forward to having his own family.

Holding back tears, his father said that as a young officer, Liu once bought a meal for someone he pulled over before guiding him home, he told the Daily News.

Liu's funeral comes five days after his partner's. His family was waiting for family to come from China.

His widow gave a brief statement on the day of Ramos' funeral, thanking members of the community for their support and sending condolences to the Ramos family.

"This is a difficult time for both of our families," Pei Xia Chen said, "but we will stand together and get through this together."

Rafael Ramos

Ramos saw his work as a police officer as a ministry and was posthumously appointed honorary NYPD chaplain

He was just hours away from becoming a lay chaplain and graduating from a community-crisis chaplaincy program before his violent death.

He left behind a wife, Maritza, and two sons, Justin and Jaden.

Vice President Joe Biden, New York Gov. Andrew Cuomo, de Blasio and Bratton delivered eulogies at his funeral.

More than 25,000 police officers from across the country and Canada attended. Officers formed long chains holding hands in prayer.