A Sydney man who was allegedly hit in the head with a scooter is fighting for his life in a Sydney hospital.

The 55-year-old was allegedly bashed in the face with the scooter by a 21-year-old man, while walking along a Marrickville footpath about lunchtime on Saturday.

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Police alleged the older man tried to intervene after an altercation sparked between the younger man and a woman, also 21.

He yelled at the older man and struck him in the head, causing him to slump to the ground, police said.

The older man was found with head injuries and taken to Royal Prince Alfred Hospital where he remained in a critical but stable condition.

Police arrested the 21-year-old man and a 21-year-old woman but later released them.

Police charged a 21-year-old man on Sunday with recklessly inflicting grievous bodily harm.

He was due before Newtown Local Court on Monday.