The victim succumbed to injuries while undergoing treatment.
According to police, the victim, identified as Deepak Das (32), was caught by the locals yesterday while he was indulging in theft at a building in suburban Sion.
A group of over 25 people started assaulting him and beat him up badly before handing him over to the Sion police.
"We immediately admitted him to the nearby Lokmanya Tilak hospital. However, he succumbed to his injuries today," a police official said.