A flash mob of 150 teens slammed into Center City yesterday afternoon like a tidal wave of stupidity, flooding the streets with chaos and fear.

The mob of mindless miscreants gathered at the Gallery mall about 4:45 p.m. for a fight that police suspect may have been arranged earlier in the day on the Internet, police said.

Mall security guards chased the marauders, only to watch them stampede through the nearby Macy's, causing $700 worth of damage, said Lt. George Ondrejka of Central Detectives.

The teens, including numerous students from Simon Gratz and Benjamin Franklin High Schools, then wreaked havoc as they moved en masse across Market Street toward City Hall, Ondrejka said.

Traffic came to a halt as the youngsters began pelting cars, pedestrians and one another with snowballs, Ondrejka said. They knocked down numerous bystanders and some members of the mob even turned on each other. Philadelphia police and several SEPTA cops swarmed the scene and collared 15 participants - 14 boys and one girl - near City Hall, Ondrejka noted.

The boys, ages 14 to 17, were charged with disorderly conduct and rioting. Ondrejka said the girl, 15, was charged with assault for kicking a teen boy in the face.

One teen, who was believed to have been a part of the mob, was hospitalized after being knocked to the ground from behind and kicked and stomped, Ondrejka said.

In the aftermath, police found "several people just crying in the streets," Ondrejka said. "They panicked. They looked up and saw this mob, and had no idea what was going on."

Ondrejka said investigators were trying to determine if the gathering had been planned on Facebook or Twitter.

He added that one teenager told detectives that the melee had been filmed by the participants and was going to be used to promote a party.

"We're very concerned about it. It's a major problem for law enforcement," Ondrejka said.

Last May 30, a flash mob of dozens of teens flooded South Street then Broad, looting a gas station, carjacking a cabdriver and injuring several pedestrians. Police said later that that incident had been arranged on social-networking sites.