Three callous teenagers set fire to a man sleeping on a park bench and then filmed him as he burnt on their mobile phones.
Police have launched a hunt for the youngsters, aged between 13 and 15, after they used a lighter to ignite the 41-year-old victim's clothes.
The man had been sitting alone with his head in his hands when the incident occurred around 8.20pm on Tuesday at Grant Thorold Park in Grimsby, Lincolnshire.
He was rescued by dog walker Wendy Watson, 30, who was horrified at witnessing the boys at first taunting and then setting him alight.
She said: "I first saw the youths nudging the man. They were shouting at him. The man was drunk - he was out of it. The youths had seen me in the park and initially pretended to be concerned for the man's welfare.
"They were calling to him, 'You all right, mate?' When I went out of sight behind a bush, they began pushing him and shouting 'wakey, wakey'. They knew he was drunk.
"I thought they were going to rob him. But when he didn't respond, they got out a lighter and set his shirt on fire. I screamed for people to help but no one came, even though there were three or four people in the park."
Wendy and another young woman helped to put out the man's burning clothes and prevented him having any serious injuries.
Humberside Police said the victim suffered burns to his back and appealed for witnesses to come forward.
A force spokesman said: "An altercation is believed to have taken place between the man and the three teenagers, which then resulted in the man's clothing being set alight."