Arthur J. Harvey, a radiation oncologist, had just finished a long day seeing cancer patients at Thomas Jefferson University Hospital and was walking from his office to his home in Center City early Friday night.

He was upbeat, enjoying the holiday atmosphere as he walked along Chestnut Street, which was still crowded at 6:15. He was approaching 13th Street when everything suddenly went black.

When he came to a moment or two later, his glasses were gone, his face hurt, and male teens were running on Chestnut toward Broad Street.

A female pedestrian told Harvey, 53, that one of the teens had come up from behind, punched him in the head, and run off. Harvey said the force of the blow caused him to black out momentarily, but he remained upright.

He found out later he must have been one of the random targets of roaming bands of teens who caused havoc near the Gallery and neighboring streets Friday afternoon and early in that evening. Some witnesses reported seeing as many 100 teens roving in the area.

Police said students from several city high schools responded to a call via Facebook to participate in a massive gathering in response to fights at the Gallery food court Wednesday and Thursday that resulted in four arrests.

Despite an increased police presence Friday, there were several assaults, such as the one on Harvey.

Police spokesman Lt. Frank Vanore said yesterday large groups of roving teens remained a problem. He said the deputy commissioner of Regional Command South had been monitoring the activity closely. "That's why there were extra police in the Gallery area."

But Vanore said the only arrests police made were for minor offenses.

Harvey did not report his attack to police. He treated his own injury, which caused swelling and left him with a black eye. As a lifelong Philadelphian, he was shaken by the experience.

"I walk around Center City, and I feel completely safe," he said yesterday. "Maybe that's not the case."

Harvey said he was baffled and troubled "that we live in a time and place where our children are plotting terroristic rampages in the heart of Center City on a medium such as Facebook."