By Scott Malone and Elizabeth Barber

BOSTON (Reuters) - Lawyers for the Boston Marathon bomber on Monday wrapped up the case to spare Dzhokhar Tsarnaev's life with testimony from a Roman Catholic nun and death-penalty opponent who said Tsarnaev told her "no one deserves to suffer" as his victims had.

The same jury that last month found Tsarnaev, 21, guilty of killing three people and wounding 264 others in the April 15, 2013, attack, later this week will begin deliberating whether to sentence him to death by lethal injection or to life in prison without possibility of release.

Since testimony began in Boston federal court in early March, the jury has heard from about 150 witness, including parents who lost children in the attack, first responders who attended to victims who had lost limbs, and Tsarnaev's Russian relatives who remembered him as a loving young boy.

But the closest they came to hearing from Tsarnaev himself was Monday's testimony from the nun, Sister Helen Prejean, 76, who described meeting him five times over the past year at the request of defense lawyers. Prejean, whose story inspired the 1993 book and 1995 film "Dead Man Walking," said she believed Tsarnaev was remorseful.

"He said it emphatically. He said no one deserves to suffer like they did," said Prejean, the public face of the New Orleans-based Ministry Against the Death Penalty and a Nobel Peace Prize nominee. "I had every reason to think that he was taking it in and that he was genuinely sorry for what he did."

The defense rested its case after her testimony.

During the trial's sentencing phase, prosecutors sought to depict Tsarnaev, an ethnic Chechen, as an adherent of al Qaeda's militant Islamic ideology who mounted the attack "to punish America" for U.S. military campaigns in Muslim lands.

Defense attorneys contended he was a willing but secondary player in a scheme driven by his 26-year-old brother.

Tamerlan Tsarnaev died on April 19, 2013, following a gunfight with police that ended when Dzhokhar inadvertently ran him over with a stolen vehicle as he sped away from the scene. Hours earlier, the pair had shot a university police officer to death as they prepared to flee Boston.

STOIC PRESENCE

Tsarnaev sat quietly through his trial, showing little sign of emotion until last week, when he briefly dabbed at his eyes as his 64-year-old aunt broke down in tears on the witness stand.

By closing with Prejean, defense attorneys ended the case with Tsarnaev's words without giving prosecutors a chance to cross-examine him, said Robert Bloom, a professor at Boston College Law School.

"I think it was a good decision. From everything we can see, he doesn't emote very much, especially in the courtroom," Bloom said. "Through Prejean, they were able to show him talking, show him caring. I thought that was really quite powerful."

It also spared Tsarnaev from being tripped up during an aggressive cross-examination by prosecutors. While prosecutors asked Prejean about her opposition to the death penalty, they stopped short of questioning her honesty, possible due to her clerical status, Bloom said.

"It's very difficult to impeach a nun of long standing," Bloom said.

CONTROVERSIAL IN BOSTON

The death penalty is unpopular in Boston, where state laws do not allow the punishment and polls show a plurality of residents would prefer to see Tsarnaev sentenced to life in prison.

The families of two of his victims have publicly urged prosecutors to drop their bid for execution, and defense attorneys have argued that a life sentence will remove him from the public eye more quickly than the appeals that surround a death penalty case.

John Oliver, warden of the maximum security prison in Florence, Colorado, where Tsarnaev would be sent if spared the death sentence, told jurors on Monday that Tsarnaev could write a book, watch television and get a college degree while in prison.

Prosecutors and defense attorneys are scheduled to make their closing arguments Wednesday following a day's recess, after which the 12 jurors will begin deliberations on Tsarnaev's fate.

Martin Richard, 8, Chinese exchange student Lu Lingzi, 23, and restaurant manager Krystle Campbell, 29, died in the bombing. The Tsarnaev brothers shot dead Massachusetts Institute of Technology police officer Sean Collier three days later.

Prejean, who initially met Tsarnaev in March as his trial was getting under way, described first setting eyes on the bomber.

"I walked in the room and I looked at his face and I remembered, 'Oh my God, he's so young.' Which he is," Prejean said. "I sensed he was very respectful, and I felt it was pretty easy to establish a rapport."

(Editing by Jonathan Oatis and Bernadette Baum)