A homeless man has been arrested after he allegedly attempted to abduct a baby boy at Bronte Beach.

A 31-year-old mother was at the beach on Monday evening at around 6pm with her seven-week-old son lying on a towel beside her, when the man approached her.

The 38-year-old man, later identified as Ronnie Ramirez, unknown to the mother, allegedly walked up to her and attempted to pick up her baby while yelling "this baby is not yours".

She attempted to push him away and the pair were reportedly involved in a struggled before lifeguards intervened.