The assault happened at about 10.50pm yesterday (Monday) in Latchmere Avenue, West Park.
A 21-year-old man was pulled out of his Audi A3 car, which was parked, by "a number" of offenders, a spokesman for West Yorkshire Police said.
He was then beaten several times with a hammer in the street.
The victim suffered head and leg injuries during the attack, and was today (Tuesday) in a serious condition in hospital.
The men ran off after the attack.
A police scene is now in place on the street as forensic examinations take place.
Detective Inspector James Entwistle, of Leeds District CID, said: “This was a very violent attack on the victim who has been left with severe injuries and remains in a serious condition in hospital.
“Our investigation is still in its early stages and we are trying to establish what the motive may have been for this attack.
“We are carrying out extensive enquiries to identify those responsible and we would like to hear from anyone who witnessed the incident or who saw anything suspicious in the area in the time immediately before or afterwards.
“We would also like to hear from anyone who has any other information that could assist us.”
Anyone with information is asked to contact Leeds District CID, quoting crime reference 13180631483 or call the independent charity Crimestoppers anonymously on 0800 555 111.