CHICAGO—Police here are girding for another weekend of "flash mob" attacks after arresting 29 people in connection with a recent rash of assaults and robberies in and around the city's tony shopping and dining district.

Twelve crimes involving large groups of young men were reported last weekend, in addition to others earlier this spring. The incidents are some of the first major problems confronting newly appointed Chicago Police Superintendent Garry McCarthy.

The attacks have received wide attention in Chicago because they have occurred around the city's affluent north side, including near the "Magnificent Mile," a Michigan Avenue strip popular with tourists.

The latest attack occurred early Thursday morning. A spokesman for the Chicago Police Department said the attack involved "multiple" perpetrators near an elevated train platform. Described in the police report as a "strong arm robbery," the group of assailants took the victim's iPad and money and punched him in the face. No arrests have yet been made in that case.

The Chicago Tribune reported the victim was Jesse Andersen, the 35-year-old brother of Smashing Pumpkins front-man Billy Corgan. Mr. Corgan sent out a tweet Thursday morning saying: "My little brother Jesse was attacked and robbed this morning by 3 men. he's ok, but if you want to send him some love."

Mr. Andersen couldn't immediately be reached for comment. The police spokesman said the department cannot confirm the identity of the victim.

In another incident last Saturday evening, Krzysztof Wilkowski, after shopping on Michigan Avenue, was sitting on his scooter a couple of blocks away checking his phone for a restaurant when he got whacked in the face with a baseball.

At first, he said, he thought it was a prank, but then he looked up and saw 15 to 20 young men approaching. "My first reaction was, 'I'm about to get robbed, what do I do?' " Mr. Wilkowski recalled in an interview.

The 34-year-old insurance company employee from a Chicago suburb grabbed the keys from his ignition and held tight to his phone. A few of the attackers dragged him off his scooter and pulled him onto Chicago Avenue where they punched him, hit him with his helmet and tried to grab his phone.

"The way we're attacking this is attack the crimes—not being satisfied locking up one or two of [the suspects], but identifying and arresting the entire group," Mr. McCarthy said in an interview.

Mr. McCarthy, who was chosen to be the city's top cop by new Mayor Rahm Emanuel and confirmed by the city council Wednesday, said more than half of the 29 people arrested were picked up Saturday or Sunday.

Six of the dozen incidents over the weekend were robberies, with five involving the same group of people, Mr. McCarthy said. Ten members of that group have been arrested. Another 19 people, mostly juveniles, have been arrested in connection with the nonviolent crimes comprising the other six incidents. In one incident, a group of 10 people robbed a student on a bus, Mr. McCarthy said.

He said officers are monitoring train stops and other areas where groups might form. He is sending additional police officers to the downtown area and deploying undercover officers. Detectives and the gang unit are also assisting.

Mr. Wilkowski said he was happy with the department's response to the 911 call he placed as soon as his attackers disbanded. He said he suffered a bruise on his face and scratches on his knees, wrist and elbow, but "not a single scratch on my phone."

Mr. Wilkowski said he plans to keep coming downtown and he doesn't necessarily want to see too many police. To fully protect people, he said, "They'd have to have police on each corner like in '1984.'"

Write to Julie Jargon at julie.jargon@wsj.com and Ilan Brat at ilan.brat@wsj.com