By Scott Malone

BOSTON (Reuters) - Three people who survived the 2013 Boston Marathon bombing on Thursday recalled seeing the first of a pair of explosions but not having enough time to react before the second homemade bomb went off, tearing off their legs.

Federal prosecutors called victims to the stand as they tried to persuade a jury to sentence convicted bomber Dzhokhar Tsarnaev, 21, to death for his role in the attack that killed three people and injured 264 others.

Heather Abbott recalled she had been waiting outside a restaurant near the race's finish line on April 15, 2013, when the first bomb went off in one of the highest-profile attacks on U.S. soil since Sept. 11, 2001.

"I saw smoke and started hearing people scream and immediately what came to mind was footage I had seen on 9/11 when the (New York World Trade Center) buildings collapsed. ... Before I could even react, the second explosion occurred," Abbott testified.

"I was catapulted through the front doors of the restaurant, which were open, and I landed on the ground in a puddle of chaos and glass and blood."

Abbott was one of 18 people who lost legs to the twin pressure-cooker bombs placed at the scene by Tsarnaev and his 26-year-old brother, Tamerlan, who died four days later after the pair shot dead a police officer.

The jury saw pictures of all 18 amputees, many of whom testified earlier in the trial, as prosecutors presented the third and final day of their sentencing-phase case in U.S. District Court in Boston.

Jurors also heard an audio recording of the reaction of the crowd outside the Forum restaurant between the time of the first and second blasts.

"Oh my God, something blew up!" a female voice screams before the second bomb goes off with a deafening bang.

Another person who lost a leg in the bombing described the aftermath.

"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 Marc Fucarile, 36, who lost his right leg.

  The same jury that found Tsarnaev guilty early this month of carrying out the attack will determine whether he is sentenced to death or to life in prison without possibility of release. This phase of his trial will include a fresh round of witnesses called by the defense.

Testifying for the prosecution, Dr. David King, a trauma surgeon at Massachusetts General Hospital and a veteran combat surgeon, told jurors the injuries caused by the homemade pressure-cooker bombs were similar to what he had seen in Afghanistan.

"The type of wounds that we see from explosive devices that are placed on the ground is fairly typical," said King, who had run the marathon that morning.

  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.

'PRIMAL' PAIN

King said Richard's death would have been intensely painful due to the severe injuries to his internal organs.

"He did not die instantaneously," the surgeon said, noting that internal injuries activate a set of pain receptors that call "a much more primal, very disturbing kind of pain."

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 Tamerlan was the driving force behind the attack, with Dzhokhar playing a secondary role.

Thursday's final witness, cancer biologist Steve Woolfenden, described setting out with his then-3-year-old son Leo to cheer on his wife, Amber, who was running.

After the first bomb went off, he turned to hustle Leo away when the second blast knocked him to the ground.

"Leo was conscious, he was alive, he was bleeding from the left side of his head," said Woolfenden, whose son, now 5, suffered a skull fracture but survived. "I thought, 'Well, let's get out of here.' And that's when I discovered my leg had been severed off."

A passerby picked up Leo, and Woolfenden looked to his left, where he saw Martin Richard and his mother, Denise.

"I saw Martin's face," Woolfenden said. "I could see a boy that looked like he was fatally injured."

(Editing by Jonathan Oatis)