A gunman burst into a baby-naming ceremony at a Pittsburgh synagogue and opened fire Saturday morning, killing 11 people during a massacre believed to be one of the worst attacks on Jews in U.S. history.

Some three-dozen worshippers had been attending services inside the conservative Tree of Life Synagogue in Squirrel Hill when the shooter, identified by authorities as Robert D. Bowers, barged in just before 10 a.m.

“All Jews must die!” Bowers screamed during the 20-minute siege, police sources told CBS Pittsburgh/KDKA.

At least six other people were wounded, including four police officers. Bowers, 46, exchanged gunfire with cops and was struck several times before he surrendered.

“It’s a very horrific crime scene. One of the worst that I’ve seen, and I’ve been on some plane crashes,” Pittsburgh Public Safety Director Wendell Hissrich told reporters, holding back tears.

The investigation will be led by the FBI in Philadelphia, and Bowers, who was in fair condition at a hospital, will likely face federal hate crime charges.

When gunfire erupted, a bris, a circumcision ceremony had been in progress. Pennsylvania’s Attorney General Josh Shapiro said the shooting happened during the portion of the rite when the child is given a Hebrew name.

The “shooter claimed innocent lives — and injured first responders — at a baby-naming,” Shapiro said.

There were no children among the dead or injured, officials said.

The two injured civilians were a 61-year-old woman who is “doing well,” and a 70-year-old man who suffered major organ damage, and who will need multiple surgeries, a hospital official said.

None of the injured officers were seriously hurt. Officials had yet to release the names of the dead by Saturday night.

Bowers, a bearded, heavyset resident of Pittsburgh — was armed with an AR-15-style assault rifle and three handguns, officials said.

He had an active license to carry a firearm, law enforcement sources told CNN, and was known to have made at least six firearm purchases since 1996, when he would have been 24 years old.

Justice “will be swift, and it will be severe,” vowed Scott Brady, US Attorney for the Western District of Pennsylvania.

The gunfire sent congregants scattering, with many calling 911 after barricading themselves inside the building, located in the city’s Jewish enclave Squirrel Hill.

Rabbi Stephen Weiss was mid-prayer when the shooting began, his son, Zachary, 26, told Buzzfeed.

“He was davening,” the son said, using the Hebrew word for ‘praying.’ “All of a sudden there was a loud boom.” His father fled home to safety, and “I gave him a big, big hug,” the son said.

Bowers continued spewing anti-Semitic slurs even as officers arrived.

“Suspect talking about all these Jews need to die,” one first responder was recorded saying on a police radio.

“The suspect is talking about killing Jews. He doesn’t want any of them to live,” another officer added.

Bowers, who was dressed in a white shirt and jeans, was injured in the police shootout and crawled on his hands and knees as he surrendered at around 11 a.m., according to reports.

He still had “a pistol in his waistband and one strapped to his ankle,” local reporter Ross Guidotti tweeted.

Bowers remained hospitalized Saturday night, with charges pending.

“This is the most horrific crime scene I’ve seen in my 22 years in law enforcement,” said Bob Jones, the FBI’s special agent in charge.

The shooting was “an attack not just on the Jewish community, but on America as a whole,” said World Jewish Congress President Ronald S. Lauder.

The Anti-Defamation League called it “likely the deadliest attack on the Jewish community in the history of the United States.”

In online posts, Bowers described himself right-wing, but virulently anti-Trump and anti-Semitic.

“Jews are the children of satan. (john 8:44)” he’d written on his bio on Gab, an alt-right version of Twitter on which he called himself “@onedingo.”

Regarding the president, he said: “For the record, I did not vote for him nor have I owned, worn or even touched a maga hat.”

Hours before the shooting, he railed against HIAS, the Hebrew Immigrant Aid Society, a Jewish non-profit that advocates for refugees worldwide.

“HIAS likes to bring invaders in that kill our people,” he posted. “I can’t sit by and watch my people get slaughtered. Screw your optics, I’m going in.”

His account was taken down shortly after the shooting, but his posts were preserved by websites including Heavy.com.

Michael Eisenberg, the immediate past president of the synagogue, and who lives about a block from the building, said he was getting ready for services when he heard about the shooting by phone.

“I ran out of the house without changing and I saw the street blocked with police cars,” he said.

“It was a surreal scene. And someone yelled, ‘Get out of here.’ I realized it was a police officer along the side of the house … I am sure I know all of the people, all of the fatalities. I am just waiting to see,” Eisenberg said.

He said officials at the synagogue had not gotten any threats that he knew of prior to the shooting.

The shooting was decried by President Trump, who called the gunman, “A madman, a wacko” and “a maniac” and urged a return to the death penalty.

“This was a baby-naming ceremony at a sacred house of worship,” he said, outraged, during a speech before the Future Farmers of America in Indianapolis.

“We condemn the historic evil of anti-Semitism and every other form of evil.”

Vice President Mike Pence joined Trump in calling for the death penalty, saying, “What happened in Pittsburgh today was not just criminal — it was evil.”

“Our hearts break for the families of those killed and injured,” said the ADL’s CEO, Jonathan A. Greenblatt, terming the attack during worship “unconscionable.”

Richard Cohen, president of the Southern Poverty Law Center, said the shooting is reminiscent of “the slaughter of nine African American worshippers at Charleston’s Mother Emmanuel Church in 2015, the killings of six Sikh worshippers at a temple in Oak Creek, Wisconsin, in 2014, and, of course, the bombing of Birmingham’s 16th Street Baptist Church in 1963 that left four young African American girls dead.”

Israeli Prime Minister Benjamin Netanyahu said the shooting left him “heartbroken and appalled.”

“May the memory of the murdered be blessed,” he said.

Additional reporting by Ben Feuerherd and Mary Kay Linge

With Post wires