Sussex Police are searching for the four boys involved in the incident which happened at “Kitty’s Field” park in Dominion Road, Worthing, at 3.30pm on September 8.
A force spokesman said: “The 13-year-old local boy was approached by four other teenage boys who asked if they could push him on the swing pictured in the above image, which he agreed to.
“Shortly afterwards, he fell from the swing on to ground, and sustained serious leg and pelvic injuries which are potentially life-changing.
“A passing member of public assisted the victim at the scene until the ambulance service arrived and took him to hospital, where he remains at this time.”
The spokesman said that the boys involved were all white and aged between 13 and 15.
One of them was described as being tanned, of medium build, with dark blond hair in a curtains style, wearing a grey Nike T-shirt.
Investigating officer David Mullins said: “We are keen to speak to the boys involved, and anyone who saw what happened, to establish the full circumstances and to determine whether any offences have been committed.”