Between 15 and 20 teenagers punched an MBTA bus driver and tried to pull him from the driver’s side window while the bus was picking up passengers in Dorchester early Saturday morning, authorities said.

The attack happened at about the same time a passenger threw her paper Charlie Ticket at the driver after she and the driver had a fare-related dispute, Transit Police Superintendent in Chief Joseph O’Connor said during a press conference this afternoon.

The 53-year-old driver was transported to Boston Medical Center, where he was treated and released for “minor injuries,” O’Connor said.

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“This is a very unusual crime,” he said. “We have seen an uptick in assaults on operators. However those are generally over fare and they usually involve one or two individuals at most. This is the first time in a long time that I can remember that a group assaulted an operator.”

The alleged assault happened shortly after 1 a.m. Saturday on a Route 16 bus that stopped to pick up passengers along Columbia Road near its intersection with Geneva Avenue, he said.

Just after the passenger threw her T pass at the driver, the group of male teens walked in front of the bus, stopping it from going forward and then attacking the driver, O’Connor said.

Some who were outside the bus tried to pull him from the window; others boarded the bus, he said. The driver was punched several times.

Authorities believe that at some point the bus driver activated emergency lights and an alarm to alert that the bus was in trouble, O’Connor said.

The group ran from the area in several directions before Boston Police and Transit Police arrived, he said. Emergency medical technicians treated the driver at the scene before he was hospitalized.

No arrests have been made.

O’Connor said the attack lasted “a brief amount of time” but did not know exactly how long.

He said investigators do not know if the fare dispute was necessarily related to the attack. No other possible motive is known. Police do not believe any weapons were used in the attack.

Detectives found the Charlie Ticket that was thrown at the driver just before the attack and are checking to see if that can help them track down the passenger who allegedly threw it.

“We’re very early on in the investigation,” he said. “This is certainly a very serious event.”

The driver, who has been with the T since May 2010, will be interviewed again, he said.

“He’s very shaken up from what I’m told right now. He’s not working as we speak but I’m sure that he will get the assistance he needs and hopefully return to work in the near future,” O’Connor said.

Police have interviewed “some witnesses,” including an unknown number of bus passengers, O’Connor said.

The bus did not have surveillance cameras on it. About one-third of the T’s bus fleet has cameras; the agency has plans to install them on more buses, O’Connor said.

Detectives are checking to see if the attack may have been caught by cameras installed on property around where it happened, he said.

During 2013, there have been 22 cases of T drivers reporting that they were assaulted or threatened. Through the same period in 2012, there were 18 such cases, O’Connor said.

He said the assaults have not been concentrated in a certain area or along a certain route.

“There’s no pattern of where these assaults occur,” he said.

Transit Police ask anyone who witnessed the attack or with information about it to contact detectives by calling 617-222-1050.