The assault took place at about 10.50pm on Monday in Latchmere Avenue, West Park.
Police said that the 21-year-old man was pulled out of his parked Audi A3 car and beaten with a hammer.
He sustained head and leg injuries and was yesterday (Tuesday) in a serious condition in hospital.
In an update, West Yorkshire Police today said that a 19-year-old man, from Leeds, has now been arrested on suspicion of causing grievous bodily harm with intent in relation to the offence.
The force is continuing to appeal for witnesses to the assault.
Anyone with information is asked to call Leeds District CID on 101, quoting crime reference 13180631483.