Barrett was hospitalized with injuries suffered Saturday night when a man beat him with a metal pipe in the parking lot of State Fair Park in West Allis, Wis., the Milwaukee Journal Sentinel reported.
Authorities said Barrett took out his cell phone when a woman who was being accosted screamed for help, and was dialing 911 when the suspect ran toward him and began striking him with the pipe. Barrett was with his family when the attack occurred.
Milwaukee Police Chief Edward A. Flynn said at a news conference police arrested Anthony J. Peters about noon Sunday. Police said Peters' criminal record includes arrests in other assault cases, the Journal Sentinel reported.
"Last night, he was a totally violent and vicious thug, and that's how he behaved," Flynn said during the news conference at the home of John Barrett, the mayor's brother, who is also the circuit court clerk for Milwaukee County.
John Barrett said the mayor's injuries were serious but doctors expect him to recover.
The woman in the incident was said to be uninjured.