Boston police rushed around the city’s downtown Wednesday night, responding to calls for help after a group of up to 20 males and females launched at least six unprovoked attacks on people on Boston Common and in the neighborhood, police said.

Police said Thursday that while there were at least six incidents, in only four of them were they able to locate victims who wanted to pursue criminal charges against their attackers. Police and prosecutors offered this summary of the four incidents:

Around 6:50 p.m. Wednesday, officers responded to 75 Boylston St., where a victim reported being attacked without provocation while walking through Boston Common by a group of males and females, police said.

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About four minutes later, a woman reported that a teenage female started a verbal argument with her while both were walking through the Common. Officers were able to track the group that included the assailant to a nearby 7-11 store, where a 15-year-old girl was issued a summons for assault, police said.

While inside the store, a man identified by police as Abdirizak Yusef, was charged with disturbing the peace when he became belligerent toward police. The 19-year-old Yusef was arraigned Thursday in Boston Municipal Court, where he pleaded not guilty and was released on personal recognizance after a judge declined to revoke his bail on a pending larceny case, Suffolk District Attorney Daniel F. Conley’s office said.

At 8:06 p.m. Wednesday, officers responded to the corner of Milk and Devonshire streets where a woman reported that she had encountered a group of male and female teenagers, and that a girl in the group had grabbed her purse. When the woman resisted, she was “violently attacked’’ by the girls, police said.

Officers identified a 15-year-old Mattapan girl as the assailant in both the Devonshire Street attack and the first attack reported on the Common. She was arrested and charged with unarmed robbery and assault and battery with a dangerous weapon, police said.

The last incident occurred at 8:23 p.m. Wednesday when a man reported he was assaulted near Otis and Summer streets and robbed of a bag carrying his cellphone and his iPad.

District attorney’s spokesman Jake Wark said a 15-year-old boy from Mattapan was arrested in that attack, and that he also allegedly robbed another man of a cellphone. Also, Wark said, a 15-year-old Mattapan girl was arrested for allegedly assaulting a woman and trying to pull her bag away from her in the same area.

The boy and the girl entered not-delinquent pleas to assault and robbery charges on Thursday and were ordered held on $10,000 bail and $1,000 bail, respectively, with conditions that they stay away from the alleged victims, Downtown Crossing, and the Common, and abide by 6 p.m. curfews, Wark said.

Advertising copywriter Joseph McNamara apparently witnessed one of the last attacks.

McNamara said he was walking on Arch Street around 8:30 p.m. Wednesday when he saw a group of about 10 people attacking a man who looked like an office worker from one of the downtown buildings.

“They were really coming down on this one guy,’’ McNamara said in a telephone interview. He said one person, a teenager with short cropped hair and red sweatshirt, was the most aggressive attacker. “I didn’t really realize what was going on until I was in the middle of it.’’

When the victim got up and tried to break away from his attackers, McNamara said, the assailants followed him as he crossed back and forth across Arch Street in search of a safe place to stop. He described the flow of attackers as “like a swarm. It actually enveloped me at one point.’’

“It was weird. They would follow him, this whole swarm,’’ McNamara said. “At one point, they just came up to me, but they were totally focused on this guy. I was right there, I could have touched him. [But] I didn’t like the odds at all. I was just standing there motionless.’’

While the teenagers swarmed down Arch Street, McNamara said he saw a male he believed to be part of the group, standing some distance away at the intersection of Arch and Summer streets. This person was watching the violence unfold, while holding closely onto a female McNamara believed was his girlfriend.

Suddenly, the male shouted, McNamara said.

“He just shouted, ‘We out!’ and everyone broke,’’ McNamara said. “They all left.’’

McNamara said once the swarm flowed past him, he rushed into a nearby 7-11 store, where he called police. When officers arrived a minute or two later, he told them what he had seen, as did the victim, who said he had been subjected to an unprovoked attack and robbed of his cellphone and iPad.

Officer Rachel McGuire, a Boston police spokeswoman, said Thursday the department did not believe the incidents were part of a “knockout game,’’ which the department considers to be an incident where a person is physically assaulted, but no other crime is committed against them. McGuire said at least some of the attacks involved robberies.

Authorities said three people were transported to area hospitals after the incidents.