(Corrects number of people who lost limbs in first and 12th paragraphs)
By Scott Malone
BOSTON, April 23 (Reuters) - One of the 18 people who lost legs in the Boston Marathon bombing on Thursday recalled being knocked to the ground by the second explosion and hearing someone yell that he was on fire.
Marc Fucarile, 36, testifying in the sentencing phase of convicted bomber Dzhokahar Tsarnaev's trial, said he had been standing in front of a restaurant near the 2013 race's finish line, saw an explosion to his left and worried that a second would follow.
"I remember a lot of pressure on my chest, felt like someone was sitting on my chest but it was from the nurse holding me down, and I remember her screaming, 'Oh shit, he's still on fire,'" said Fucarile, whose right leg was amputated on the scene.
Fucarile's testimony came on the third day of trial's sentencing phase. The jurors who convicted Tsarnaev will determine whether the 21-year-old ethnic Chechen is sentenced to death or life in prison without possibility of parole.
Tsarnaev was found guilty early this month of killing three people and injuring 264 in the April 15, 2013, attack, as well as fatally shooting a police officer three days later as he and his brother prepared to flee the city.
Fucarile wheeled himself into the courtroom in a wheelchair and glared at Tsarnaev while he waited for proceedings to start. Tsarnaev avoided his gaze.
In making their case that Tsarnaev should get the death penalty, federal prosecutors have called a series of witnesses to describe the permanent gaps in their lives after the loss of their loved ones.
Eight-year-old Martin Richard, 23-year-old Chinese graduate student Lingzi Lu and 29-year-old restaurant manager Krystle Campbell were killed in the bombings. Massachusetts Institute of Technology police officer Sean Collier was shot to death in his police cruiser.
Prosecutors have said that Tsarnaev read and followed bomb-making instructions found in al Qaeda's "Inspire" magazine and left a note suggesting the attack was an act of retribution for U.S. military campaigns in Muslim-dominated countries.
Defense attorneys, who are due next week to begin making their case that Tsarnaev should get a life sentence rather than death, are expected to argue that 26-year-old Tamerlan was the driving force behind the attack, with Dzhokhar playing a secondary role.
Tamerlan died following a gunfight with police hours after Collier's murder.
Fucarile, one of 18 people who lost legs in the attack, has been among the more outspoken victims.
In December, he angrily confronted a group outside the courthouse who claimed the bombing was a hoax, pointing to his prosthetic right leg as evidence of its reality. (Editing by James Dalgleish)