Slain NYPD cop Jason Rivera was given a solemn final farewell on Friday as his grieving brother told the thousands of officers gathered for his funeral that the rookie’s “first love was policing.”

Rivera, 22, was posthumously promoted to detective first grade during the service at Manhattan’s historic St. Patrick’s Cathedral — one week after he was murdered in the line of duty.

“He was obsessed with his career in law enforcement,” his emotional older brother, Jeffrey Rivera, said during his eulogy.

As top New York officials joined Rivera’s family inside the church, a staggering “sea of blue” stretched several blocks along Fifth Avenue as cops huddled outside in the snow to pay their respects.  

Dominique Rivera, his childhood sweetheart whom he married in October, tragically revealed the couple had been fighting the day he died.

When they left the apartment together as Rivera headed to work, Dominique told mourners she called an Uber — instead of having him drive her — because she didn’t want to keep arguing.

“You said it might be the last ride I give you,” Dominique recalled. “I said no … and that was probably the biggest mistake I ever made.”  

Rivera and his partner, Wilbert Mora, were fatally wounded in Harlem just hours later.

“He gave his life defending his fellow New Yorkers,” Mayor Eric Adams told mourners.

“The hearts of 8.8 million people are reaching out in mourning today,” Adams added. “We know he’s always with us and this city will become a better place because of his sacrifice.”

NYPD Commissioner Keechant Sewell said Rivera “was everything the NYPD and the city needed him to be.”

Gov. Kathy Hochul, Attorney General Letitia James and Sen. Chuck Schumer were among the other dignitaries present to pay their respects.

Also among the mourners were the widow and parents of Wenjian Liu, the NYPD cop executed alongside his partner in Brooklyn in 2014. Liu’s family were hugged by several officers before making their way into the church.  

Rivera’s service was only open to invited guests, but everyday New Yorkers still gathered as snow fell outside the church to honor the slain officer.

“I just have to be here for him and his family,” Isaac Krinsky, 61, of the Lower East Side, told The Post. “He gave his life not just for me but for all New Yorkers. They put their lives on the line for all residents of the city and pay the ultimate price.” 

Miguel Melendez, 60, and his son Mason, 10, traveled from their home in Baldwin Harbor in Nassau County to claim a spot outside the cathedral. 

“It’s a tragedy what happened,” Melendez said. “If it didn’t touch your heart, you’re not human. 

“I woke my son up at 5:30 a.m., pulled him out of school and said, ‘You’re coming with me to see how someone paid the ultimate sacrifice for trying to unite our community.’ This hatred towards cops has to end. It is unspeakable what has happened.”

Jim O’Neill, an NYPD detective, said: “I think he’s definitely a New York hero. He gave it his all. I admire the man that he was and I’m feeling great sorry for his family.

“Things like this just break my heart to see what is happening. It’s just a very challenging time right now. I’m here to support the job that I love so much and the people that I admire and respect.”

Sam Pirozzolo, a 58-year-old Staten Island optician, said bail reform and defund the police had turned “the people who protect us into villains.”

“How many more people have to die? Police officers killed, people pushed in front of trains, little girls shot in the head — the city is burning,” he said. “Hopefully today will be a turning point.”

Following the funeral Mass, a private cremation ceremony was held at Ferncliff Cemetery in Westchester County.

Members of the public were invited to gather on several overpasses that overlook the Thruway in Yonkers as the officer’s body was escorted to the cemetery.

Thousands of mourners paid their respects during a public wake on Thursday when Rivera’s casket, wrapped in an NYPD flag, was escorted into the cathedral.

Rivera was killed when deranged career criminal Lashawn McNeil ambushed him and his partner, Officer Wilbert Mora, during a domestic violence call in Harlem on Jan. 21.

Mora, who died Tuesday from his injuries, is due to be mourned at a funeral at St. Patrick’s next week.

The suspect was shot by a third officer, rookie Sumit Sulan, during the attack on Rivera and Mora and died from his wounds earlier this week.