Defence lawyers have asked a jury to sentence Boston bomber Dzhokhar Tsarnaev to life in America's "super-max" federal prison, portraying a lost teenager controlled by a radical older brother.

"As awful as his crime was, life in prison faced with what he has done is a better choice for everyone," lawyer David Bruck said in his opening statement at the penalty phase of Tsarnaev's trial on Monday.

The 21-year-old former student was found guilty this month of carrying out the 2013 bombings that killed three people and wounded 264 in one of the deadliest attacks in the US since 9/11.

The immigrant of Chechen descent was convicted on all 30 counts related to the bombings, the murder of a police officer, a carjacking and a shootout while on the run in April 2013.
Bruck showed the Boston courtroom a picture of America's only federal "supermax" prison in the wilds of Colorado, where he said Tsarnaev would live out the remainder of his days in obscurity.

"You will punish him and protect society at the same time," Bruck said.

There would be "no martyrdom" for the killer who claimed the attacks were to avenge US wars in Iraq and Afghanistan.

"He will be securely locked there, he will never be heard of again," Bruck told the court.

He stressed over and over the influence that 26-year-old Tamerlan waged over his younger brother, then 19. Tamerlan was shot dead by police while on the run after the bombings.

He was aggressive, "failed everything" and became radical under the influence of his mother Zubeidat Tsarnaeva who "turned to fundamentalist religion and made sure Tamerlan followed", Bruck said.

The defence is expected to take two weeks to make their case.

Tsarnaev, thin and pale, showed no emotion. Even during the most harrowing testimony, he has refused to look at his victims, many of whom have walked on prosthetic limbs to the witness box.

Tsarnaev's convictions leave the jury only two sentencing options: the death penalty or life without parole.

An opinion poll published in the Boston Globe newspaper Monday found that just 19% of residents in Massachusetts believe Tsarnaev should be put to death. In Boston, the number fell to 15%, with nearly 66% of city residents favouring a life sentence, the paper said.