First responders and law enforcement swarmed the area surrounding the Annunciation Church just after 8:30 a.m. following reports of heavy gunfire, authorities confirmed.

A male gunman was later found dead, law enforcement sources told The Post.

An initial emergency police alert described “a man dressed in all black and armed with a rifle” had opened fire and there were “at least 20 victims.”

Local police haven’t confirmed how many people were injured.

At least six children were being treated at Children’s Minnesota — a pediatric trauma hospital, according to officials.

Hennepin Healthcare, which has Minnesota’s largest emergency department, said it also was caring for patients from the shooting.

Minnesota Gov. Tim Walz said he was praying for our kids and teachers whose first week of school was marred by this horrific act of violence.”

“I’ve been briefed on a shooting at Annunciation Catholic School and will continue to provide updates as we get more information,” Walz said in an X post.

City officials confirmed the shooter had been “contained” and there was “no active threat to the community at this time.”

“I’m monitoring reports of horrific violence in South Minneapolis,” Mayor Jacob Frey said on X.

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“I’m in touch with [Police] Chief O’Hara and our emergency response team has been activated. We will share more information as soon as we can. Please give our officers the space they need to respond to the situation.”

The school, which is located next door to the church, is for kindergarten through eighth-graders.

Students returned to class on Monday.

A weekly “all-school mass” was scheduled to take place at 8:15 a.m., according to the church and school’s website.

Scores of local and state police, federal Bureau of Alcohol, Tobacco and Firearms agents and other authorities were spotted in the vicinity of the scene.

A reunification zone was set up near the school, while others were being urged to stay away “to allow emergency personnel to help victims.”