Police on Monday were searching for two people who attacked a homeless man as he slept in a park and set his legs on fire.

The 30-year-old victim, whose name was not released, was being treated for burns Monday at Massachusetts General Hospital, officials said.

Mayor Thomas M. Menino promised that the attackers would be tracked down and arrested.

"We're going to find you," he told reporters. "We're going to lock you up. We're going to hope the jury and the judge throw the book at you."

The homeless man told police he was awakened early Sunday by the men kicking him in Langone Park in the city's North End. He drifted back to sleep but the men returned, drenched him with a flammable liquid and set his legs on fire, police said.

A 911 caller reported flames in the park, and firefighters found the man wrapped in a blanket after he had ripped off some of his burning clothes.

The park is a short walk from a cluster of downtown bars.

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.

"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.

In the overwhelming number of cases, the attackers are teenagers, according to the national report.

Last August, a 40-year-old homeless man died in Boston after he was beaten; two teenagers were charged with manslaughter.

In Fort Lauderdale, Fla., a homeless man was beaten to death the same night a surveillance camera captured two people assaulting a homeless men with a baseball bat. A third homeless man was bludgeoned nearby with a golf club. Three teenagers have pleaded not guilty to charges of murder and attempted murder.