Luis Pacheco, 18, Khalil Smith, 15, and Daniel Walker, 17, friends of Donnell McFarland, 18, who was shot last month. They were arrested on the way to his wake.

Correction Appended

The police officers hopped from their vans and cars with shouts of “Hands up,” “Don’t move,” and “Get on the ground.” Someone in the crowd of young people yelled, “Nobody run” — and nobody did, witnesses said. The teenagers were frisked, forced up against a fence or a wall, or pushed to the asphalt.

Asher Callender, 19, helped organize the meeting of teenagers in a park. The idea, he said, was that they would all take the subway together to Mr. McFarland’s wake.

Those watching said the mood was almost subdued as the handcuffs went on, the loudest sound the whir of a television news helicopter hovering above. “None of us understood what was going on,” said Dana Hollis, whose teenage daughter was arrested. “Everything just happened so fast.”

Thirty-two young people, the youngest 13, were arrested the afternoon of May 21 in Bushwick, Brooklyn. They had been walking as a group to the subway, which they planned to take to Coney Island for the wake of Donnell McFarland, 18, who had been fatally shot a week earlier.

The police, already fearing retaliatory violence, say the teenagers were exchanging gang signs, wearing T-shirts with a gang name and bounding atop cars when they were arrested. Parents and teachers of the group and witnesses said that they were no more boisterous than any group of teenagers would be in similar circumstances, and that they did not see any youths atop cars.

The charges are misdemeanors: unlawful assembly and disorderly conduct. No drugs or weapons were found, and there were no injuries to those arrested or to the police. The officers did not draw their guns. Yet this roundup of Brooklyn teenagers and young people has gotten widespread attention.

Interviews with those arrested, their parents, witnesses who did not know the teenagers, as well as accounts provided by the Police Department and the Brooklyn district attorney, provided contradictory versions of events. But they correspond in one aspect: The arrests were part of a police operation that unfolded with precision.

Undercover officers circled in unmarked cars; a police captain monitored the teenagers gathering; and blue-and-white vans and buses cut off Putnam Avenue in both directions at a key moment, trapping the teenagers less than a block into their journey.

“Once the kids hit Irving, the police came from everywhere,” said Lisa Guerrero, 52, who lives nearby and saw the group gather and head up the block. “I was like: ‘What happened? Why is this happening?’ “

Paul J. Browne, the Police Department’s chief spokesman, said, “The police were being responsive to community leaders who warned that the group was poised for trouble after a week of murder, shootings and fistfights between two rival gang factions in Bushwick.”

On May 15, Mr. McFarland was shot in the head at Linden Street and Knickerbocker Avenue by James Kelly, 16, the police said. Mr. Kelly was soon arrested and charged with murder and criminal possession of a weapon. Friends of both said the shooting was the climax in a string of violent events involving Mr. Kelly, a onetime friend of Mr. McFarland’s turned enemy.

The shot echoed for blocks.

“We were on the basketball court, and we all kind of froze,” said Asher Callender, 19. Someone ran into the park, crying, using Mr. McFarland’s nickname: “They killed Freshh.”

The police say the murdered teenager was the leader of the Pretty Boy Family, which they describe as a subdivision, or “set,” of the Bloods gang. But those who knew Mr. McFarland and are familiar with the Pretty Boy Family described it as a tight group of friends who like to dance and hang out, not a gang. The police say the Pretty Boy Family had been at odds with James Kelly’s gang, the Linden Street Bloods, another Bloods subdivision, for some time. Both sets frequent the Hope Gardens housing project.

Word of Mr. McFarland’s death spread from the neighborhood streets into neighborhood schools.

“I didn’t have a single class that whole week where I didn’t have two or three people in my class crying,” said Tabari Bomani, a social studies teacher and college counselor at Bushwick Community High School, where many students knew Mr. McFarland. Dozens of them met with grief counselors, school officials said.

Mr. McFarland’s wake was set for the following Monday at a funeral home in Coney Island. Officials at the Bushwick high school allowed students to sign out for the day if parents signed a permission slip.

Mr. Callender said that many students wanted to attend, but that he was one of the few who knew the way to the funeral home. So he spread the word: Meet at Putnam Park between noon and 12:30 the day of the wake, May 21. They would gather, walk up Putnam, and head for the subway station.

Meanwhile, police were connecting the dots in a yearlong investigation into the Pretty Boy Family and a recent rash of gang violence.

The police said a Pretty Boy Family member was shot in the foot two weeks before Mr. McFarland’s death. Later, they said, there was a confrontation between William Gonzalez, who had been feuding with Mr. McFarland, and a man they believed belonged to the Pretty Boy Family.

The same day, Jakai King, whom the police described as a member of the Linden Street Bloods, was attacked by members of the Pretty Boy Family, the police said. Two days later, they said, he was attacked again, this time stripped down to his underwear and sent running down the street.

It was in this atmosphere of attacks and revenge, Mr. Browne said, that the police received reports that a gang would be “mustering at the park” the day of the wake and that there would be violence. Community leaders warned the police in the 83rd Precinct in Bushwick and the 60th Precinct in Coney Island that Mr. McFarland’s rivals had said that they would shoot anyone wearing a T-shirt memorializing him.

Ms. Hollis, 40, her 15-year-old daughter and two of her nieces joined Donna Seabury and her two daughters, 12 and 16, at the park. Mr. Callender said he went to the school that morning to retrieve the school permission slip he left with the assistant principal. He then headed to the park. He was early, one of the first students to arrive. His friends slowly trickled in.

The teenagers were to wear similar T-shirts, bearing Mr. McFarland’s picture and words like “R.I.P. Freshh,” to the wake.

Luis Pacheco, 18, said he went to the print shop that morning to pick up his $14 T-shirt. After meeting a friend, they went to the park. It was just about 12:30 p.m.

Others recounted similar stories: rushing to school to get slips, waiting for their parents to walk with them to the park, meeting friends to travel together.

Zezza Anderson, 18, said teenagers sat in small groups or off by themselves. “Everyone’s sad. We’re sad; we’re grieving,” Mr. Anderson said. “No one was being rowdy. Just chilling, waiting for everyone to show up. We’re trying to make sure we don’t leave anybody behind.”

Just after 1 p.m. the students walked from the handball courts to the macadam path that leads to the street.

Capt. Scott Henderson, of the 83rd Precinct, was one of the officers doing surveillance. In his report, he wrote that the teenagers greeted one another with gang hand signs, wore gang bandannas and shirts with a gang name on them, and gathered near a wall covered with gang slogans. Mr. Browne said last week that since they believed Mr. McFarland was in a gang, the police considered “Freshh” a gang name.

The police said the group then left the park and took over Putnam Avenue, stopping traffic, frightening pedestrians and hopping onto parked cars. “It’s when he sees that group grow in size and start walking on cars and forcing others to go into the street is when he called for the arrests,” Mr. Browne said of the captain. “It’s not like they had a plan where they were going to go to the park and arrest people.”

Some witnesses, including some parents, said the teenagers were behaving peacefully. The Brooklyn district attorney’s office said witnesses saw unruly behavior, including walking on cars, but Charles J. Hynes, the district attorney, would not provide specifics of those accounts.

Several owners of cars that were parked on the block said they did not notice any damage to their vehicles afterward.

Hector Polonia, 52, was sweeping the sidewalk in front of United Cleaners, where he works as manager, on Irving Avenue near Putnam, when he saw the group crossing Putnam. Then he saw the police move in. “They weren’t making any noise or anything,” Mr. Polonia said. “They were acting like a normal bunch of teenagers.”

Ms. Guerrero was sitting in Putnam Park. “They didn’t get on any cars,” she said of the teenagers.

Those under 16 were quickly released. The six female mourners in the group were given summonses for disorderly conduct. The remaining young men were run through the system, charged with disorderly conduct and unlawful assembly. Most remained in jail overnight, some as long as 36 hours.

“They seemed more interested in asking us questions about the murder than telling us why we were arrested,” Mr. Callender said. “They just kept asking us stuff like, ‘Who has the guns?’ and ‘Who is going to strike next?’ But they were giving me information about people that I didn’t even know. They knew more than we did.”

Several of the teenagers said they were interrogated about the Pretty Boy Family. Some said the police pulled out a big black binder labeled “P.B.F.” with photos of people from the neighborhood.

“At first I thought they were going to question us, then let us go,” said Mr. Pacheco. “But then I could see the sun going down from my cell. I got upset. And people were crying. Some people were throwing up.”

The last members of the group were released on May 23. The next day, a news conference was held at the headquarters of Make the Road by Walking, a community rights organization in Bushwick.

More than 50 students, including many of those who were arrested, demanded an apology from the Police Department.

The district attorney has offered to give those arrested community service assignments in return for guilty pleas. So far, none have accepted.

Al Baker, Michael Brick and Daryl Khan contributed reporting.

Correction: June 27, 2007

A picture with an article on Sunday about the circumstances behind the mass arrests of Brooklyn youths who were on their way to Coney Island for the wake of a slain teenager on May 21 carried an erroneous credit in some copies. The photograph of Asher Callender, 19, one of the arrested teenagers, speaking at a community meeting two weeks after the arrests was by Ramin Talaie for The New York Times — not by Chester Higgins Jr. of The Times.