PHILADELPHIA -- Activists opposed to Democratic presidential candidate Hillary Clinton shoved open an imposing security fence on Wednesday evening as President Barack Obama addressed delegates nearby, triggering the most intense standoff with police this week near the Democratic National Convention.

“Don’t do it,” a young woman pleaded with a man in his 20s moments before the fence crashed open. He donned a bandana anyhow and moved swiftly toward the barricade, which flung open as though it was a pair of french doors.

There was little warning that this would happen, despite loud chants of “open the gates!” shortly before and tepid banging against them by supporters of Vermont Sen. Bernie Sanders, runner-up to Clinton for the Democratic presidential nomination.

Moments before the gate-crashing, the crowd of hundreds was most enthusiastic about the chant “Hell no, DNC! We won’t vote for Hillary!”

Police officers on the other side of the fence surged forward when it opened toward them. Activists shoved it back, and the barricade swung back and forth. Some activists were immediately taken into custody. Others may have been in the chaos that followed as a police line formed on the protest side of the fence.

The U.S. Secret Service said in a statement early Thursday that seven people were in federal custody relating to the fracas. They are charged with violating 18 U.S. Code 1752, entering a restricted area, the same charge leveled against four people who jumped over the fence on Tuesday. It's possible others face non-federal charges.

After order was vaguely restored, the more-radical activists in the crowd set an American flag on fire. That happened on at least two occasions Tuesday, provoking small verbal skirmishes, and earlier in the evening on Wednesday with no objections. But in the tense aftermath of the gate fight, a woman wearing a long dress and a shirt for the progressive protest group Philly.fyi jumped on top of the fire and attempted to stomp it out. Her dress caught fire, to the horror of everyone watching.

Shortly after she set herself on fire, a small group including police gathered nearby, lathering an ointment on her legs after the fire was extinguished.

Tension mounted with opposing bullhorns and chants. More-moderate supporters of Sanders slowly cleared out, re-forming ultimately to sit in the middle of Broad St. and sing “we are in it together” over and over.

Members of the bandana-wearing group that re-formed in the aftermath, many of whom consider themselves anarchists, said they had come across the country to protest.

One man said “maybe three” of his ideological brethren managed to force open the fence. He said the loosely organized group is unlikely to do much on Thursday, the final day of the convention.

As chaos subsided, members of the bandana-clad group formed a circle and spoke against “white privilege” and chanted “black lives matter!” A leader of the chants, who declined to provide his name, spoke Tuesday during the anti-police, anti-Clinton Black DNC Resistance March, where he said he was born in Mexico.

On Thursday evening, peace will again be tested as members of the Revolutionary Communist Party plan to burn a U.S. flag at 6 p.m. near the site of the melee. The group did so in Cleveland during the Republican National Convention last week, resulting in a rush of police, who arrested 18 people at or near the scene under circumstances bound to attract detailed legal analysis. Though also a far-left group, the RCP did not lead Wednesday’s activism here.

“If you’re here for the revolution, stay here!” a man wearing a bandana called as crowds thinned before midnight Wednesday.