Police say she and her boyfriend, Chris Kelly, 28, were set on as they walked through the park near their home in Bexhill, East Sussex. Detective Superintendent Paul Westwood said: "All we can tell you is what the companion of this young lady has told us.
"He [Mr Kelly] said he and the lady were walking along the footpath when he thinks there was a group coming towards them and for no apparent reason one of them doused her with what he took to be fluid and the next thing she was alight. He tried to put out the flames, dragged her off the side of the footpath on to the grass area where there are a number of puddles and tried to immerse her in water as much as he could where she was burning. At this stage we do not have any witnesses."
Mr Kelly, who is also in hospital, suffered burns to his hands.
Detectives are working on the theory that a gang of youths were responsible.