Boston Marathon bomber Dzhokhar Tsarnaev's lawyers urged a jury Monday to spare his life, portraying him as "a good kid" who was led down the path to terrorism by his fanatical older brother.

David Bruck delivered the defence's opening statement in the penalty phase of Tsarnaev's trial, saying there is no punishment Tsarnaev can get that would be equal to the suffering of the victims.

"There is no evening the scales," Bruck said. "There is no point in trying to hurt him as he hurt because it can't be done."

Tsarnaev, 21, was convicted of 30 federal charges in the twin bombings that killed three spectators and wounded more than 260 other people near the marathon's finish line on April 15, 2013. He was also convicted of killing an MIT police officer during the Tsarnaev brothers' getaway attempt.

This stage of the trial will determine whether he is executed or spends the rest of his life behind bars.

Bruck urged the jury to sentence the defendant to life in prison without the possibility of ever being released.

"His legal case will be over for good, and no martyrdom, just years and years of punishment," the lawyer said. "All the while, society is protected."

Bruck focused heavily on Tsarnaev's now-dead older brother, Tamerlan, portraying him as a volatile figure who led the plot. He said Tamerlan was "consumed by jihad" and had "power" over an admiring Dzhokhar.

Bruck contrasted Tamerlan with Dzhokhar, saying Tamerlan was loud and aggressive, got into fights, failed at everything he did and never held a steady job, while Dzhokhar was a good student in high school, was loved by his teachers there, had many friends and never got in trouble.

"He was a good kid," the lawyer said. But he said Tsarnaev started going downhill in college, when his parents divorced and returned to Russia, and he was left with Tamerlan as the de facto head of the family.

Bruck said the defence will not claim that Tamerlan forced Dzhokhar to participate in the attack. But he said "if Tamerlan hadn't led the way," the bombing wouldn't have taken place.

Tamerlan went to Russia for six months in 2012 to join jihadi fighters and returned to the U.S. even more radicalized, Bruck said. He said Russian relatives will describe how "fanatical" he seemed during that visit.

Bruck said Dzhokhar grew up amid turmoil and instability. He was born in the former Soviet republic of Kyrgyzstan, then moved from place to place with his parents and siblings before settling in the U.S. in 2002 when he was 8, the attorney said.

Bruck showed the jury photos of the Supermax prison in Colorado, where Tsarnaev would probably serve his sentence if he were given life instead of the death penalty.

Tsarnaev's existence would be austere, with most of his time spent in solitary confinement and his communications with the outside world severely restricted, Bruck said. His only exercise would be for brief periods outside in a small cage.

Tsarnaev was a 19-year-old college student at the time of the bombing. His brother, 26, was killed days after the attack when he was shot by police and run over by Dzhokhar during a chaotic getaway attempt.

The first two witnesses called by the defence Monday described two incidents at a local mosque when Tamerlan Tsarnaev became angry and interrupted prayer services.

Loay Assaf, an imam, said that in one of those incidents, in January 2013, Tamerlan became furious when Assaf likened the Rev. Martin Luther King Jr. to the Prophet Muhammad. Assaf said Tamerlan took a "fighting stance" and began pointing at him and shouting.

"He said, 'You're a hypocrite,' insulting me with this," Assaf said.

Prosecutors portrayed Tsarnaev as an unrepentant killer who gave the finger to the security camera in his jail cell three months after his arrest. The startling photo and video were shown to the jury last week.

Bruck downplayed the gesture, saying Tsarnaev was "acting like an immature 19-year-old."