BUFFALO — A gunman wearing military gear and livestreaming with a helmet camera opened fire with a rifle at a Buffalo, New York, supermarket in what authorities called “racially motived violent extremism,” killing 10 people and wounding three others Saturday before he was arrested, authorities said.

Police officials said the gunman, who also wore body armor in addition to military-style clothing, pulled up Saturday afternoon and opened fire with a rifle at a Tops Friendly Market, adding the shooting was streamed via a camera affixed to the helmet.

“He exited his vehicle. He was very heavily armed. He had tactical gear. He had a tactical helmet on. He had a camera that he was livestreaming what he was doing,” the city police commissioner, Joseph Gramaglia, said at a news conference.

Mr. Gramaglia said the gunman shot four people outside the store, and three of them died. Inside the store, a security guard who was a retired Buffalo police officer fired multiple shots at the gunman and struck him, but the bullet hit the gunman’s bulletproof vest and had no effect, Mr. Gramaglia said. He said the gunman then killed the security guard.

Video showed the suspect walking into the supermarket and shooting several other victims inside, according to authorities.

Officials said 11 of the victims were Black and two are white. The shooting happened in a predominantly Black neighborhood a few miles north of downtown Buffalo.

“This is the worst nightmare that any community can face, and we are hurting and we are seething right now,” Buffalo’s mayor, Byron Brown, said at a news conference. “The depth of pain that families are feeling and that all of us are feeling right now cannot even be explained.”

The gunman was identified as Payton Gendron of Conklin, a New York state community about 200 miles southeast of Buffalo, two law enforcement officials told the Associated Press. The officials were not permitted to speak publicly on the matter and did so on the condition of anonymity.

The suspect was being questioned Saturday evening by the FBI, one of the officials said, and was expected to appear in court later Saturday.

At the news briefing, the Erie County sheriff, John Garcia, called the shooting a hate crime.

“This was pure evil. It was straight up racially motivated hate crime from somebody outside of our community, outside of the City of Good neighbors … coming into our community and trying to inflict that evil upon us,” Mr. Garcia said.

Elsewhere, the NAACP president, Derrick Johnson, released a statement in which he called the shooting “absolutely devastating.”

“Our hearts are with the community and all who have been impacted by this terrible tragedy. Hate and racism have no place in America. We are shattered, extremely angered and praying for the victims’ families and loved ones,” he added in a statement.

The Reverend Al Sharpton called on the White House to convene a meeting with Black, Jewish and Asian “to underscore” that the government is “escalating its efforts against hate crimes.” In a tweet, Sharpton said that “leaders of all these communities should stand together on this.”

Braedyn Kephart and Shane Hill, both 20, pulled into the parking lot just as the shooter was exiting. They described him as a white male in his late teens or early 20s sporting full camo, a black helmet, and what appeared to be a rifle.

“He was standing there with the gun to his chin. We were like what the heck is going on? Why does this kid have a gun to his face?” Mr. Kephart said. He dropped to his knees. “He ripped off his helmet, dropped his gun, and was tackled by the police.”

Tops Friendly Markets released a statement saying, “We are shocked and deeply saddened by this senseless act of violence and our thoughts and prayers are with the victims and their families.”

Police closed off the block, lined by spectators, and yellow police tape surrounded the full parking lot. Mayor Brown and the Erie County executive, Mark Poloncarz, were at the scene late Saturday afternoon, gathered in a parking lot across the street from the Tops store and expected to address the press.

At the White House, the press secretary, Karine Jean-Pierre, said President Biden is receiving regular updates on the shooting and its aftermath.

“The president has been briefed by his Homeland Security advisor on the horrific shooting in Buffalo, New York, this afternoon. He will continue to receive updates throughout the evening and tomorrow as further information develops,” she said, adding the president and first lady were praying for the victims and their loved ones.

More than two hours after the shooting, Erica Pugh-Mathews was waiting outside the store, behind the police tape.

“We would like to know the status of my aunt, my mother’s sister. She was in there with her fiance, they separated and went to different aisles,” she said. “A bullet barely missed him. He was able to hide in a freezer but he was not able to get to my aunt and does not know where she is. We just would like word either way if she’s okay.”

Governor Hochul tweeted that she was “closely monitoring the shooting at a grocery store in Buffalo,” her hometown. She said state officials have offered help to local authorities. The Erie County Sheriff’s Office said on social media that it ordered all available personnel to assist Buffalo police.

Attorney General Garland was briefed on the shooting, a Justice Department spokesman, Anthony Coley, said.

The shooting came little more than a year after a March 2021 attack at a King Soopers grocery in Boulder, Colorado, that killed 10 people. Investigators have not released any information about why they believe the man charged in that attack targeted the supermarket.

By CAROLYN THOMPSON, MICHAEL BALSAMO and DAVE COLLINS. John Wawrow and Eric Tucker contributed to this report.