One man was smoking a cigar Saturday night on a bench in the Streeterville neighborhood when his evening was interrupted by a group of young men who robbed and beat him. Another man was parking his motor scooter outside Northwestern University's downtown campus when the same group apparently made him their next victim, police said.

Police believe the group of about 15 to 20 youths in their mid- to late teens also was responsible for two more attacks moments later along the lakefront.

At least seven people thought to be part of the group were arrested Saturday in connection with the robberies and mob action, a crime that has become a problem in Streeterville, the Gold Coast and the Magnificent Mile, authorities said.

Five youths were charged early Monday in connection with a series of robbery events that occurred about 8:30 p.m. Saturday in the area of the Chicago Avenue and Lake Shore Drive, police said.

Dvonte Sykes, 17, of the 7500 block of South Normal Avenue was charged with two felony counts of robbery and one count of mob action violence to person and property.

Two 16-year-old boys also were charged in a juvenile delinquency petition for the same incident. One boy was charged with a felony count of robbery and one count of mob action violence to person and property, police said. The other boy was charged with one count of misdemeanor theft, control of stolen property of less than $300 and mob action. The youths were not named because they are juveniles.

Two other youths, Trovolus Pickett, 17 and Derodte Wright, 18, were charged in connection with a robbery on the 300 block of East Chicago Avenue, police said.

Pickett of the 8400 block of South Dorchester Avenue was charged with three felony counts of robbery, this includes the robbery committed on the North Lake Shore Drive. Wright of the 3500 block of South State Street was charged with one count of felony robbery for the incident on East Chicago Avenue, police said.

In mid-May, Chicago police started implementing strategies to address mob action incidents, which are often coordinated via text messages or social networking websites, in downtown and neighborhoods near it. Police said the youths in many cases come downtown using mass transit.

Police, however, have stressed that this phenomenon, involving large groups of teens, is not unique to Chicago. Philadelphia has had problems with mobs of teenagers, who also used social networking technology, assaulting pedestrians and vandalizing property since at least last year, according to media reports.

The latest incidents in Chicago apparently started about 8:20 p.m. Saturday, when a 68-year-old man from Washington state was sitting on a bench smoking a cigar in the 300 block of East Chicago Avenue as the young men approached, authorities said. The group beat him, robbed him of an iPad and BlackBerry and fled the scene, authorities said.

What authorities believe was the same group also approached a 34-year-old man as he parked his motor scooter outside a building on Northwestern's downtown campus along East Chicago Avenue, just west of Lake Shore Drive. One of the males in that group threw a baseball at the victim's face and knocked him to the ground, and several others allegedly punched and hit him multiple times.

Authorities said the group attacked two more men moments later on a nearby bicycle path. One was a 42-year-old man visiting from Japan, who was beaten up and had his iPod stolen from him. The other was a man in his 20s who was punched and had his bike, wallet and iPhone taken.

None of the victims was seriously hurt.

On Sunday afternoon, the buzzing of a Chicago police helicopter flying back and forth along Lake Michigan could be heard nearby at Lake Shore Park, at Chicago Avenue and Lake Shore Drive. Police also said they are monitoring surveillance cameras on Michigan and Chicago avenues, State Street and the beaches.

Nellie Maldonado and her boyfriend, Pete Tirado, who operates a hot dog stand outside the park's field house, said that during the weekdays they have seen packs of 10 or more teens occupy the benches several feet away from where Tirado sets up shop. The way they talk and how they dress, Tirado said, makes it appear as if they were part of a street gang.

"When I see these (benches) get loaded with (those teens), I pick up and leave," said Tirado, who also had his stand at the same spot last year.

Tirado and Maldonado were working at the stand Saturday, but they left during the afternoon thunderstorm. They said they heard on Sunday about at least one of the attacks from customers.

"I'm not going to run away from here, but I don't stay after a certain time," said Tirado, who added he usually leaves the park around 7:30 p.m. at the latest.