A woman has suffered serious head injuries after she was bashed by a homeless man in Melbourne’s CBD.

It’s alleged the man, of no fixed address, punched the 54-year-old woman in the back of the head while she was walking along Exhibition Street.

Nearby tradesman rushed to help her.

She was taken to The Alfred in a serious condition.

Police have arrested a 44-year old man of no-fixed address who remains in custody and are urging anybody who witnessed the random attack to contact Crime Stoppers on 1800 333 000.