Widow Peixia Chen holds a photo of slain New York Police Department Detective Wenjian Liu as his casket leaves the funeral home in the Brooklyn borough of New York on Sunday. Carlo Allegri / Reuters

Waves of dark blue stood out on a gray, somber day as more than 20,000 police officers and civilians turned out Sunday for the funeral of Wenjian Liu, the New York Police Department officer killed in the line of duty last month.

The funeral at the Aievoli Funeral Home in Brooklyn started at 11 a.m. as uniformed police officers from across the United States filled several avenues along 65th Street.

Liu's family members stoically recalled details of his life and described him as caring, hard-working and committed to family.

"Wenjian, you are the best son," said his father, Wei Tang Liu. "You are the best husband. Also, you are our police officer and our best friend." The man described losing his only child as "the most difficult time" and was unable to continue his speech after breaking up.

"To me, he is a soul mate," said Liu's crying widow, Pei Xia Chen. "Wenjian is an incredible husband, son, co-worker and friend, my best friend." The couple had been married only since September.

"I do not know why there is so much evil and heartache in our world," said FBI Director James Comey, representing the US government. "Our obligation is to try to make something good out of tragedy, so the evil is not allowed to hold the field, so the evil is not allowed to win the day."

"All of our city is heartbroken," said New York City Mayor Bill de Blasio.

Liu is believed to be the first Chinese-American police officer killed on the job in New York City, according to The New York Times. He was laid to rest in Cypress Hills Cemetery.

Liu, 32, and his colleagueRafael Ramos, 40,were fatally shot while sitting in their patrol car on Dec 20 in Brooklyn byIsmaaiyl Brinsley, who then shot himself to death in a nearby subway station.

Brinsley, 28, had posted on Instragram earlier about his plan to avenge the deaths of Eric Garner on Staten Island and Michael Brown in Ferguson, Missouri, in incidents involving police this summer.

At the end of the funeral, Liu's casket, covered with the blue, green and white NYPD flag, was carried into a hearse as officers saluted while somber bugle music played. Three helicopters flew across the sky shortly afterward. The hearse, together with dozens of police motorcycles, moved slowly eastward along 65th Street and was watched by thousands of police officers. They stood in formation until the car disappeared from sight. Some were wiping their eyes.

"They are doing a difficult job every day," retired police officer John Magin told China Daily. "It's getting more difficult throughout the country."

Magin said that ordinary people often see police making arrests but do not see the hardships they endure in protecting communities, including "protecting people protesting them".

Magin was referring to a series of nationwide protests triggered by the deaths of Garner and Brown. De Blasio was considered overly sympathetic to the protesters in the eyes of many police officers, and some turned their backs to the mayor during both funerals.

Both the mayor and the police should "make sure that they go forward together", said US Representative Peter King of New York. "The tragedy can be an opportunity to find a way for the mayor to go with the police."

The wake for Liu was held Saturday at the same funeral home on a bleak, rainy day.Media were denied access per the Liu family's request, but people who paid respects saidBuddhist rituals were performed, with candles, incense and lotus lights. Family members sat in a room receiving condolences.

"This is a really tragic story," New York Governor Andrew Cuomo said after the wake. "This is really pointless. It had nothing to do with them," he said of Liu and Ramos. "They did nothing wrong. ... It was pure and random hatred."

Martin Golden, a state senator and former NYPD officer, said: "We are going to have a hearing at the Senate to see how we can help the Police Department get the equipment and training they need."

"We are here to support Liu's family and NYPD officers," said Hannu Tarjamo, one of about 20 officers from the Los Angeles Police Department who attended the funeral. "When it happens here, it happens to us. It is a loss to us as well, and officers across the country."

Residents and businesses around the funeral home also showed their support by hanging blue ribbons outside their homes.

"It is the time when police officers need public support most," said Sharon Li. "We came to the wake to show our support towards the NYPD."

De Blasio attended the viewing Saturday, but did not give a public speech.

Liu, a native of Guangdong province in southern China, came to the US at age 12 with his parents to pursue the American Dream. He decided to be a police officer after the 9/11 terrorist attacks in 2001, and joined the NYPD in 2007 after serving as an auxiliary police officer.

Wu Yachang, a retired NYPD detective, felt a special bond with Liu.

"We came to the US and then joined the police, with the same dream and career," said Wu, who served more than two decades. "When I knew his story, it was just like looking at me 30 years ago."

"It is a very sad loss and pity," said Guan Ziyuan, a representative ofthe Long Zhang Association. "He was protecting us and our communities, and he was doing good deeds."

Donations have poured in since the officers' deaths.The Stephen Siller Tunnel to Towers Foundation, a non-profit organization helping veterans and firefighters, has announced it would payoff the mortgagesof both Liu's and Ramos' families. More than$600,000 out of a goal of $800,000 had been raised by Jan 2. Chinese communities and the New York Daily News also have been raising money for the Liu family.

On Dec 31, de Blasio announced that West 6th Street and Ridgewood Avenue in Brooklyn would be renamed "Detective Wenjian Liu Way" and "Detective Rafael Ramos Way", respectively. The blocks are where the two officers' homes are.

Lu Huiquan in New York, Xinhua and the AP contributed to this story.