NEW YORK — A solemn line of fellow police officers and everyday New Yorkers, some standing in the cold for three hours Thursday, filed inside St. Patrick’s Cathedral to offer their prayers for slain NYPD Officer Jason Rivera.

The casket bearing the 22-year-old officer reached the venerable Midtown church around 11:30 a.m., with family members arriving prior to an afternoon viewing open to the public.

Photos of Rivera from throughout his young life decorated the altar around an open casket adorned with red roses while his widow and parents sat together as the mourners poured in from the Manhattan streets.

“I had to come,” said Yvonne Miller, her voice a bit shaky after traveling from the Bronx for the wake. “I had to come — I didn’t have a choice. Because this is our city, and he was part of our city, and he belongs to us. And he takes care of us and look at what it cost him. I feel like we’re family, you know?”

“It was just so sad,” she continued. “He’s a baby. He belonged to us all. It could have been any one of our sons ... Just seeing him lying there, I have grandchildren his age.”

Mayor Eric Adams, a former NYPD captain, was among the early arrivals for the somber farewell six days after Rivera’s fatal shooting, with officers from Philadelphia, Virginia and Toronto traveling to pay their respects.

Officer Wilbert Mora, 27, was also killed in the deadly confrontation with an unhinged ex-con, with a wake and funeral scheduled to honor the slain hero next week.

Rivera, in his dress blue uniform with gold buttons and hat, was flanked by an American flag and the NYPD flag. Prayer cards from the viewing bore the officer’s badge number 25738 and offered a message of optimism written in Spanish: “Nothing is impossible for those who have faith.”

“Sad, of course,” said mourner Dennis Horkenbach, 51, of Lower Manhattan. “To see a young guy like that dead, and it’s senseless. I’m a human being, so it was sad ... I’ve never done anything like this before.”

A dozen floral bouquets paid homage to the young officer killed after just 14 months on the job where he earned the respect of his colleagues.

Rivera and Mora were ambushed and shot Jan. 21 while responding to a domestic dispute in a Harlem apartment, targeted by a gunman whose social media posts included anti-police and anti-government rhetoric.

Scores of solemn colleagues stood along Fifth Avenue in homage to their slain brother, with an NYPD helicopter whirring overhead and a lone police bagpiper playing “Amazing Grace” as the casket was brought inside the cathedral.

The assembled officers saluted his flag-draped casket on its way past on a brisk January day. The line of mourners stretched the length of a city block — and doubled back two more times inside the barricades set up for crowd control.

Gunman Lashawn McNeil, 47, burst out from a bedroom in his mother’s Harlem apartment and starting blasting with a stolen Glock 45 handgun. The armed assailant, after shooting Rivera, stepped over the mortally-injured officer’s body and gunned down Mora before a third officer took McNeil down.

Rivera died a short time later on the night of Jan. 21 while Mora survived for four days — with his family opting to donate his two kidneys, liver, pancreas and heart for transplants, officials said. Their killer was mortally wounded and died three days later.

Mora, a four-year NYPD veteran, will be honored next week in St. Patrick’s Cathedral at a Tuesday wake and a Wednesday funeral.

Mourner Carmen Figueroa, 67, of the Bronx, said additional measures to protect police were needed after five NYPD officers were shot in the first month of the year. The other three officers, shot in three separate incidents, survived.

“Something’s got to be done,” she said. “These families, they’re not going to be the same anymore, because they were just young, starting their careers.”