Zac Brown incited an unruly Boston crowd at a Fenway Park concert and security failed to protect a blind woman from the mob ... so she claims in a new lawsuit.

Karen Perry says she went to the concert last August and was seated in the disabled section. Zac came down to press the flesh with his fans, and they stormed the disabled section ... around where Zac was standing.

Security stepped in to control the situation but according to Perry only made things worse. One of the guards lifted the metal gate, causing it to lodge in Perry's armpit. She says she was so badly injured she can no longer use her white cane to navigate.