Philadelphia police yesterday made their first arrest in the beating of a man during a rampage on South Street two weekends ago.

Stephen Lyde, 21, was arrested at his residence on the 5400 block of Sansom Street in West Philadelphia at 11:30 a.m., said Capt. Laurence Nodiff, commanding officer of South Detectives.

Many of the people who went to South and Broad Streets on the evening of May 30 appear to have been responding to text messages and online social networking sites, creating a type of spontaneous gathering known as a "flash mob."

Nodiff said Lyde was not tracked down through the Internet. Detectives expect to make more arrests soon.

The victim, who police say is 54, was riding his bicycle home from his job at the Wistar Institute in University City around 11:30 p.m. when he was confronted by eight males at Broad and South, Nodiff said.

"They brutally and viciously beat him to the ground, to unconsciousness," Nodiff said.

The victim was taken to Hahnemann University Hospital in critical condition with brain swelling, a skull fracture, facial and rib fractures, and internal injuries, Nodiff said. He remained in the hospital yesterday.

The Wistar Institute declined to comment, citing employee confidentiality.

That night, mobs were involved in three other major incidents.

A cabdriver was assaulted and robbed around 12th and South Streets, and his taxi was taken and crashed a couple of blocks away. A paralegal working for the district attorney and a companion were pulled from a vehicle on Broad and assaulted and robbed. And a man standing outside a McDonald's on Broad was hit in the head with a metal object. The wound required six stitches.

The assault at Broad and South was the most brutal, Nodiff said.

Lyde is charged with aggravated assault, robbery, riot, and related offenses.

Since the rampage, Nodiff said, patrols have been stepped up in the South Street area, and police will have "zero tolerance" for any criminal activity, no matter how minor.

Nodiff asked anybody with information, photos, or videos of the incidents to contact South Detectives at 215-686-3013.

Contact staff writer Robert Moran at 215-854-5983 or bmoran@phillynews.com.