A homeless man sleeping in a park woke up early Sunday morning to two men kicking him in the stomach, police said.
He drifted back to sleep after the assault, but his attackers weren't finished.
The two men returned, drenched the 30-year-old homeless man with a flammable liquid and set his legs on fire, police said. A 911 caller reported 5-foot flames in Langone Park in the city's historic North End.
Firefighters thought a garbage can was ablaze when they arrived around 2 a.m.
"On further investigation they discovered it was a middle-aged gentlemen in a blanket," fire department spokesman Scott Salman said.
The homeless man, whose name was not released, had ripped off some of his clothes to stop the burning.
"He was shivering and was partially dressed," said Boston police spokesman John Boyle.
The victim was treated at Massachusetts General Hospital for burns to his legs. The suspects ran away, and crossed the nearby Washington Street Bridge headed toward Charlestown.
The victim told firefighters that he was attacked by two "youths," who they assumed were teenagers, Salman said. Boyle described the suspects as about 30 years old.
No arrests have been made and police gave no indication of what provoked the attack.
The park is a short walk from a cluster of downtown bars that close at 2 a.m.
Nationally the number of assaults against the homeless has risen dramatically since 2002, according to a recent report by the National Coalition for the Homeless. In 2005, 73 homeless people were assaulted nationwide and 13 died, the report said.
Last August a 40-year-old homeless died in Boston after police say he was beaten by two teenagers. Thomas Grealish, 17, and Ryan Leonard, 15, have been charged with manslaughter.
"It's the perception that it's someone insignificant that doesn't matter," said Emmanuel Smith, a counselor at the Pine Street Inn, a Boston homeless shelter. "They are laying in the street. They think they don't have any family and no one cares about them. That's not the case."
In the overwhelming number of cases of violence against the homeless, the attackers are teenagers, according to the national report.
The phenomenon made national news in January when a surveillance camera captured two teenage men in Fort Lauderdale, Fla., beating a homeless man to death with baseball bats. The video led police to the suspects.
In Maine, a Portland teenager was charged with aggravated assault for allegedly stabbing a homeless man in the throat last March. The victim, Michael Powers, 35, underwent a tracheotomy following the attack, police said.