A grandfather has reportedly been left "brain dead" in hospital after he was bashed unconscious in front of his family on Christmas night by a gang of youths armed with baseball bats and metal bars.

Anthony Clark was bashed outside his home on Esther Crescent in Mooroolbark, just before 11pm, in an incident police believe may have involved as many as eight people.

The man's brother-in-law, Mark, told 3AW the family was outside their home when they were attacked.

“They knocked my sister out, and had my niece – from what I understand – by the hair,” he said.

One of his family members, a 25-year-old woman, managed to lock herself inside the home with her two-month-old baby and a toddler, according to 3AW.