A West Philadelphia man - charged in one of the first "flash mob" incidents in Center City, during which a bicyclist was beaten unconscious - pleaded guilty Monday in an agreement that will keep in prison for five to 20 years.

Stephen Lyde, 23, pleaded guilty to aggravated assault, conspiracy, and riot in the May 30, 2009, assault on Thomas Fitzgerald, who was yanked from his bicycle and beaten during the melee at Broad and South Streets.

Lyde told Common Pleas Court Judge Susan I. Schulman that he wanted to accept responsibility for what had happened to Fitzgerald, and he apologized to his victim.

"I forgive you," replied Fitzgerald, 56, in a statement that moved some in court to tears, said Assistant District Attorney Jennifer Mitrick.

In a phone interview Monday afternoon, Fitzgerald, who is white, recalled an incident in high school when he had an altercation with a black teen who held him against a locker and other black students intervened to help Fitzgerald out.

His assailants in the flash mob assault were black, but Fitzgerald said that didn't matter to him.

"I think the sentencing was fair," Fitzgerald said. "I never held a grudge" against Lyde.

The assault occurred about 11:30 p.m. as Fitzgerald, who lives in South Philadelphia, rode home from his night-shift job and stopped his bike to watch a mob that appeared to be getting out of control. He remains disabled and unable to work.

In the telephone interview, Fitzgerald recalled getting grabbed and hit. "I felt like I was going under water," he said. He doesn't recall what happened next - that he was transported to the hospital, had a seizure in the emergency room, and then was placed in a medically induced coma.

Fitzgerald said the behavior of the mob that night continued to puzzle him. "What kind of world do you want? What kind of city do you want? Freedom is one thing. Responsibility is another thing," he said.

The incident was the first of several of what police described as "flash mobs" convened by teens and young adults summoned through texting and online social-messaging sites.

The incidents led to an increased police presence on the South Street corridor on weekend nights.

Lyde was among the first adults to be arrested after a bystander identified him as one of five to seven men who stood around Fitzgerald after the beating watching him have a seizure on a car hood.

Lyde was put near the scene by a security video camera inside a Sunoco convenience store at Broad and Catherine Streets. Lyde, well over 6 feet tall and dressed in a prominent plaid shirt, was seen on the video among a large group of young people who rushed in and ransacked the store, causing $6,598 in damage.

Lyde was arrested on the front steps of a house in the 5900 block of Osage Avenue, waiting for a delivery of clothing bought online from Neiman Marcus using Fitzgerald's stolen credit card.

Lyde pleaded guilty last year to charges involving the Sunoco and credit card incidents but, until Monday, had planned to go to trial on the assault charges.

Contact staff writer Joseph A. Slobodzian at 215-854-2985 or jslobodzian@phillynews.com.