The turbulent events in Oakland last week that resulted in about 150 arrests appear to have been largely spurred on by a small group of protesters, including a petite 25-year-old with a bicycle and a booming voice.

Many students had been lured to Schenley Plaza Thursday by the prospect of spotting President Barack Obama drive past on his way to a G-20 summit dinner at Phipps Conservatory. A few others had been playing Frisbee, doing yoga on the grassy plaza or heading home from the campus library when they were sidetracked by the spectacle of busloads of police arriving to block the way to the conservatory.

The tenor changed when a dozen protesters arrived fresh from being gassed by police in Lawrenceville. Many of them had been rounded up by 25-year-old Crystal Hoffman, an activist, poet, performance artist and community college instructor who taunted police and led the crowd in chants.

"Whose streets?" she yelled from her bicycle, prompting the response "Our streets!" as she urged students to break through a line of yellow police tape stretched across the road.

Pitt student Ian McMullen, 18, of suburban Harrisburg, was among many who got drawn into the adrenaline rush of the tumult. He ended up wearing a piece of the police tape across his chest like a sash.

"People are just exercising their right to assemble," Mr. McMullen said as a police officer used a megaphone to order students to leave.

Ms. Hoffman, meanwhile, needed no megaphone to get her message across.

Some bystanders said activists, small in number, were using students as pawns to gain a larger presence in their effort to disrupt the G-20 summit.

Ms. Hoffman doesn't see it that way.

"I gave them some key points of why they should be angry, and they really seemed to respond to it," she said in an interview yesterday. "There were students who were genuinely convinced that what the G-20 stands for is unethical and unjust, and they didn't know what to do. ... They didn't know how to express themselves or how to demonstrate their First Amendment rights."

She said she wasn't trying to start a riot, just to inspire people to speak up for themselves.

"I wasn't the only one. Credit goes to the other protesters, too. It takes a lot more than one person chanting," Ms. Hoffman said. "You can only lead a chant for so long. I completely lost my voice the next day."

It's unfortunate that the penalties were so severe for joining in -- or even for just observing from close range, Ms. Hoffman said.

Some students who came just to watch found themselves without a way out when police units began moving in from all directions.

Others were pushed by police onto Forbes Avenue. Those who lost their balance from the shove were pinned to the ground and handcuffed by baton-wielding officers.

Police had been "obviously in attack mode" from the beginning, Ms. Hoffman said. The presence of so many armed officers slapping riot sticks against their palms in unison "put students in the mood that there's something wrong here and they should do something about it," she said.

Ms. Hoffman takes no responsibility for students being arrested or subjected to pepper spray and rubber bullets.

"That's not the way it should have been. Those students were treated extremely unfairly by overly aggressive police who wanted to justify their presence," she said.

The crowd mostly comprised rebels without causes who chanted things such as "Go Pitt!" and "Let's go Steelers!"

Late Thursday evening police began to advance toward students and launch smoke canisters, some students began to get frightened and leave, while others came streaming into the plaza to view the spectacle.

"I don't know why the police are doing this," said freshman Steff Forbes, 19, of West Chester. "We're not being violent. We're just taking pictures and looking."

Her friend Chris Hoffman agreed.

"This is not a threatening crowd at all," said Mr. Hoffman, 18, of Buffalo, N.Y. "[Pepper spray] is in use and they're charging at us on horses. We're scared."

Despite the fear, Mr. Hoffman didn't immediately leave.

"It's history here. We're never going to be living in a city where the G-20 is ever again," he said.

Others who had been peaceful observers stayed because police action provoked them to participate in the protest instead of just watch.

It's difficult not to feel your rights have been violated when police are firing eye irritants, rubber bullets and smoke at students on their own campus, several in the crowd said.

"It was the police that incited a lot of this. It was police overreacting," said junior Becca Litwin of Philadelphia. "We were just students hanging out."

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