A line of police officers and a line of lawyers faced off at Pittsburgh Municipal Court yesterday, quarreling over why more than 100 people were arrested in the streets of Oakland on the final day of the G-20 summit.

Police argued that they confronted a hostile crowd in Schenley Plaza on the evening of Sept. 25 and gave multiple dispersal orders before beginning mass arrests.

But attorneys for at least a dozen arrestees, who were in court for preliminary hearings, countered that police exaggerated the threat from the crowd and officers at the scene failed to give instructions on where to go to avoid arrest.

"Can you give us the name of any individual who was disorderly in Schenley Plaza prior to the dispersal order?" attorney Jon Pushinsky asked police Lt. Ed Trapp.

"No, I can't," the lieutenant replied.

More than 70 hearings related to the summit were scheduled yesterday, starting at 8:30 a.m. and ending at 8 p.m. At least 36 people agreed to perform community service to clear their records.

But a group swept up in the Oakland arrests decided to fight charges of failure to disperse and disorderly conduct. For more than an hour, five defense lawyers fired back-to-back questions at Lt. Trapp and Officer William Friburger, of the Pittsburgh SWAT team, about the police operation last month.

Lt. Trapp, who served as assistant operations commander on the ground in Oakland, said police decided to mobilize a large force of officers in riot-control gear for the Schenley rally, hoping to avoid a repeat of the previous night, when windows at a dozen businesses were smashed.

Chief Nate Harper told Lt. Trapp they would let protesters stay in the plaza until 11 p.m., when the park officially closes. If anyone remained, officers would tell them to leave.

"Our plan for this entire thing was not to arrest. It was to disperse," the lieutenant said.

When Lt. Trapp arrived in Oakland at about 9 p.m., there were as many police officers in body armor as there were people in the plaza, about 100 to 150 on each side.

But the crowd started to become unruly, with bottles flying in the air and some protesters charging toward the police line, Lt. Trapp said. Police also received reports that several protesters were hiding in the wooded area at southern end of the plaza, behind the line of officers.

A state police helicopter flying overhead spotted people in the woods, Lt. Trapp said. Police called in their "red team" to provide protection for the rear of the line of riot-control officers.

At 10:42, police issued their first dispersal orders using two Long-Range Acoustic Devices, or LRADs, which can project clear recorded messages up to a quarter-mile away, even on the lowest volume setting.

The crowd moved out of the plaza, and small groups broke off in different directions.

Officer Friburger said some protesters taunted police, shouting, "[Expletive] you guys. We live here. You don't!" They would back away from police, but they refused to leave the streets.

About "90 percent" of the crowd did leave, using a variety of exit routes, he said. He advised one group, who told him they were from Minnesota and wanted to obey police dispersal orders, to head toward South Dithridge Street.

At 11:07, after receiving an order from Deputy Chief Paul Donaldson, police started making arrests, encircling a crowd of about 60 people who had gathered on the lawn in front of Pitt's Cathedral of Learning.

Throughout the hearings, lawyers probed every aspect of the police response, and neither Lt. Trapp nor Officer Friburger hid their annoyance with some questions.

Mr. Pushinsky, who was representing three arrestees, asked Lt. Trapp how many bottles he had seen flying through the air.

"I didn't sit there and count," Lt. Trapp said.

A public defender asked if police announced specific dispersal routes.

"I figured they were college students. They could figure it out on their own," the lieutenant said.

Another attorney asked Officer Friburger where the students were supposed to go.

"Anywhere," he said.

"How do you know they're protesters?" Mr. Pushinsky asked.

"They're not leaving. They're protesters," Officer Friburger responded.

At one point, a police officer sitting in the corner of the courtroom raised his hands in exasperation and shouted, "Lawyers!"

A dozen other officers also sat in the courtroom during the hearings, awaiting their turn to testify against individual defendants.

"Next?" Lt. Trapp said after one lawyer was finished questioning him, prompting laughter.

By the early evening, three district judges had found at least 11 people guilty of summary offenses, including one of Mr. Pushinsky's clients, 19-year-old Peter Vankoughnett, a student at Oberlin College in Ohio. He was ordered to pay a $300 fine and court costs.

The hearing process was much smoother for people who agreed to perform community service.

"I just want it to be over with as quickly as possible," said Joseph McKenna, 19, a Pitt sophomore who said he was leaving a coffee shop when he was arrested in Oakland. "I'm very angry about the whole thing."

District Judge Anthony W. Saveikis told one arrestee, 18-year-old Ryan Martini, to volunteer with senior citizens.

"Make it worth your while, OK? Don't just go through the motions," he said. "It gives you good perspective."

"Yes sir," Mr. Martini replied.

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