As a 10:00 p.m. enforced curfew loomed, the mood on the streets in Baltimore shifted from positivity to a tense stand-off with law enforcement that was quickly dispersed. (Whitney Leaming/The Washington Post)

Hours before rioting erupted on the streets of West Baltimore, a warning of the violence and fires to come began circulating on social media.

It was a photo, taken Saturday, of a throng of angry demonstrators, two of them standing exultantly on the roof of a police car. The crowd was protesting the April 19 death of Freddie Gray, 25, who suffered a severe spinal injury while in Baltimore police custody, an injury that has yet to be fully explained. Superimposed on the photo, an ominous message:

All High Schools Monday
@3 We Going To Purge

From Mondawmin, To The Ave,

Back To Downtown #fdl

A purge?

In a 2013 movie called “The Purge,” starring Ethan Hawke, crime is legal in America from sundown to sunrise one night annually. That’s how the country keeps lawlessness in check for the rest of the year: by allowing citizens to roam the streets and vent their rage for a 12-hour stretch, to rob, rape and kill with impunity. The police take the evening off.

Events leading to Gray’s arrest and hospitalization

Whether “The Purge” inspired a call to mayhem Monday in Baltimore, whether the violence was orchestrated or began organically, isn’t clear. But Baltimore police were aware of the “purge” posting on social media, and they reacted to it during the day.

What eventually followed was a night of sirens and flames, of flying bricks and shattered glass, of fighting, looting, bloodshed and more than 250 arrests.

“Mondawmin” referred to Mondawmin Mall, on Liberty Heights Avenue in West Baltimore. Near Frederick Douglass High School, it’s about a half-mile north of Shiloh Baptist Church, where Gray’s funeral was held Monday morning.

The mall is a popular after-school gathering place for students. Also, the Maryland Transit Administration’s Mondawmin bus station is there. And the Mondawmin subway station is downstairs. So a lot of teens who use public transportation walk toward the mall after classes at Douglass and other schools.

Police began to mobilize, and the public took notice.

Like others, Carrie Wells, a Baltimore Sun reporter, tweeted a photo of the “purge” post Monday. “Re the high school ‘purge,’ this is the . . . post I’ve seen circulating,’ Wells wrote at 11:31 a.m. Several return tweets followed.

Among them: “this is whipping around nextdoor and facebook,” one of her followers replied. “People are freaking out. It seems like hype? Any evidence?”

Tweeted another: “ ‘The Purge’ is a frikking movie. Somebody send BPD a link to Snopes,” a Web site about urban legends.

“My wife is at BCCC,” still another Twitter user wrote at 11:39 a.m., referring to Baltimore City Community College, not far from the mall. “Please tell me if I should tell her to leave...Please answer.”

Police Commissioner Anthony W. Batts said about 300 officers were sent to the mall to meet students from two high schools who used social media to spread word of a “purge.” Police shut the subway station at 2:57 p.m.; it wasn’t clear when the “loop” with the bus stops shut down.

Although classes at Douglass end at 4:05 p.m., a spokeswoman for the school system said some students “chose to leave early.” Around 3 p.m. (“@3,” as the posting put it), as scores of youths headed toward the mall, a battalion of officers stood waiting. Maybe some students had a purge on their minds. Maybe, in the Baltimore tinderbox of anger and distrust, some of them reacted irately to a phalanx of men and women in blue. Or maybe something of both.

Shouts and taunts quickly escalated. Batts said that officers were assaulted with “bricks and stones and sticks.” Several were injured, he said, including one who was rendered unconscious.

Then the youths dispersed, and many of them headed a mile south to the area of Pennsylvania and West North avenues. There, Batts said, they surrounded a police car, briefly trapping an officer. The neighborhood descended into anarchy. A CVS store and a check-cashing outlet were looted. And the CVS, along with a police van and a cruiser, were set ablaze.

The “purge,” if that was the intention, roared into the evening.

“WE ARE ASKING ALL PARENTS TO LOCATE THEIR CHILDREN AND BRING THEM HOME,” police tweeted at 5:40 p.m. Then, at 6:33 p.m.: “The group at Pennsylvania Ave & North Ave has cut one of the fire department’s hoses while they are attempting to put out a fire.” Said Batts later in the night: “This was not exercising First Amendment rights. . . . These young people know right from wrong. This is our city. We have to live here.”

After dark, violence flared in other pockets of West Baltimore.

Brian Woodyard Jr., 32, said that just before 9 p.m., he swung a machete at about 50 would-be looters who tried to break into his workplace, the Old Clubhouse Beer & Wine store. “It’s the only thing I have,” he said. “I can’t pull out a shotgun.”

Meanwhile, Marvin Warfield, 29, who lives in West Baltimoreand is a sound engineer for a church, watched flames leap from a car. The fire crackled and grew, shooting yellow flares into the night. “What builds up with youth only leads to explosion,” Warfield said.

Another car burst into flames near Pennsylvania Avenue and Presstman Street. “Get back! Get back!” a man on the corner yelled at onlookers. “That was just the battery popping. The gas tank will explode next!” As he spoke, looters hustled from one store in the neighborhood to another, arms loaded with junk food and other items. A young woman, with a trash bag over her left shoulder said, “Everything is free.”

Warfield watched in dismay.

“They don’t even need the stuff,” he said. “It’s just an opportunity.”

A purge?

Whatever it was, said Batts, it had nothing to do with Gray or any righteous expression of animosity toward the police. “This was not a protest,” he said, as Monday turned to Tuesday, as the mobs thinned but the fires still burned.

Officers had been “pulled in opposite directions” by rioters who “outflanked us” and “outnumbered us,” Batts said.

He called it “a very trying and disturbing day.”

Lynh Bui and Hamil R. Harris contributed to this report.