Nearly 15,000 mostly Asian-American protesters rallied in Brooklyn Saturday for former NYPD Officer Peter Liang, claiming that the rookie cop was a “pawn” of anti-police politics and was wrongly prosecuted for a tragic accident.

The crowd filled Cadman Plaza Park, with many carrying signs with slogans like, “One tragedy, two victims” and, “Scapegoating won’t bring peace.” Many placards bore Martin Luther King Jr.’s photo and quote, “Injustice anywhere is a threat to justice everywhere.”

Many in the crowd maintained Liang was prosecuted because he is a minority, while white cops involved in fatal incidents against African-Americans were not.

Protesters handed out petitions demanding Brooklyn Supreme Court Justice Danny Chun, who presided over the case, “set aside the illogical verdict handed down by the jury” or at least sentence the cop to probation only.

Liang, 28, a Chinese-American, faces five to 15 years in prison after he was convicted on Feb. 11 on manslaughter and misconduct charges in the fatal November 2014 shooting of 28-year-oldAkai Gurley in a stairwell of the Louis Pink Houses in East New York.

Liang fired his gun after hearing a noise while conducting a vertical patrol in a darkened stairwell. The bullet ricocheted off a cinder-block wall and struck Gurley, an unarmed black man, in the chest.

“What happened could have happened to any one of us,” said retired cop Joe Murray, now a criminal-defense attorney. “I’ve been in that situation, and it’s very scary. He [Liang] is absolutely being used as a scapegoat. This is their opportunity to try to redeem themselves through a conviction.”

Businessman Don Lee, who is running for Sheldon Silver’s Assembly seat in a lower Manhattan district that includes Chinatown, called for Brooklyn DA Kenneth Thompson “to state publicly to the sentencing judge that Peter Liang did not intend to kill Akai Gurley,” noting the prosecutor said as much “in an interview with Chinese TV.”

Local Chinese activists have begun crusading against Thompson, whose prosecution they call “persecution.” They handed out leaflets with Thompson’s face crossed out.

Liang, who was fired from the force upon the verdict, will be sentenced by Chun on April 14.

One of his attorneys, Robert E. Brown, who received scattered boos as he took the stage, told the crowd Liang is “not doing well.”

“There’s some false rumors that he tried to commit suicide,” Brown said. “He’s extremely upset. He’s distraught. Seeing this type of support is helping his overall demeanor get through this.”

One of the protest organizers, Phil Gim of the Coalition of Asian Americans for Civil Rights, said Liang’s family is looking for a new lawyer to handle his appeal.

“I think if Peter Liang was white, he would have lots of support,” Gim said. “The city, the Police Department, his union all abandoned him and hung him out to dry. Not one person from the PBA stood behind him. If we didn’t come out to support him, it would be a slap in his face.”

The Brooklyn protest — the largest of at least 40 across the nation — began with the national anthem and Pledge of Allegiance and included a moment of silence for Gurley.

Liang’s mother, Fenny Liang, speaking in Mandarin through a translator, said she “understands the pain of losing a loved one.”

“They’re deeply sorry for the tragic accident,” the translator added.

One popular protest sign bore the slogan, “Stop injustice to minority officers,” and paired pictures of Liang and Kizzy Adonis, the black NYPD sergeant charged last month with failure to supervise at the scene of Eric Garner’s July 2014 death in Staten Island. Daniel Pantaleo, the white officer who took Garner down, was not indicted in the case.

“There are two white police officers who didn’t even have a trial, in Ferguson and Staten Island,” said protester Tiu Wu, 24, a student who came from China six months ago. “That makes Chinese people very nervous. It is obvious it was an accident.”

A counterprotest of about 20 people was held nearby by the Black Lives Matter movement. They held signs that read, “Jail killer cops,” and, “Justice for Akai Gurley.” They would not speak to The Post.

Eric Allens, 19, was not part of the Black Lives Matter group, but disagreed with the Liang rally.

“I feel the verdict is absolutely correct. Justice is already served,” he said. “Saying it’s racism against Liang is basically nonsense. Peter Liang is not being used as a scapegoat. He needs to serve time because he committed a murder.”

Similar protests took place in Dallas, Denver, Chicago and Miami. An estimated 5,000 people marched in downtown Philadelphia. Hundreds gathered on Boston Common and at the Washington Monument.

Thousands more rallied in the Chinatown districts of San Francisco and Los Angeles.

Tweets with photos from the rallies said they were in part an effort to get Asian-Americans more politically engaged.

“Register your anger by registering to vote!” tweeted Chung Seto, a longtime Chinatown resident and former executive director of the New York State Democratic Committee, who rallied in Brooklyn.

Karlin Chan, executive director of the Chinese Action Network (CAN), last week reiterated that the death was accidental. “This wasn’t an intentional shooting,” he said.

Chan, who stood by Liang throughout the trial, said the group is seeking a candidate to run against Thompson next year.

A Facebook page dedicated to Thompson’s defeat, titled Vote Out Ken Thompson 2017, features a link to CAN’s Web site and calls the black DA a “racist.”

Additional reporting by Abbie Gepner