Image copyright Reuters

The Boston Marathon bomber is remorseful, a defence witness testified as his lawyers finished their case to spare his life.

Forty witnesses testified for Dzhokhar Tsarnaev, including anti-death penalty advocate Sister Helen Prejean.

She testified on Monday that Tsarnaev told her of the bombing victims: "No one deserves to suffer like they did."

The April 2013 bombings killed three people and injured more than 260.

Tsarnaev has expressed genuine regret and sorrow over the bombings, Sister Prejean said.

Prosecutors pushed to exclude her testimony, but a judge decided to allow it.

Image copyright Getty Images Image caption Sister Helen Prejean has been a vocal opponent of the death penalty

The Roman Catholic nun began prison ministry in 1981 in New Orleans and corresponded with Patrick Sonnier, a death row inmate who had been convicted of killing two teenagers.

Sister Prejean wrote a best-selling book on her experiences called "Dead Man Walking," which was later turned into a movie.

Prosecutors will now call rebuttal witnesses. Both sides will deliver closing arguments before the jury decides Tsarnaev's fate.

Tsarnaev's lawyers have admitted he played a role in the attacks but said that his older brother, Tamerlan, was the driving force.

The defence emphasised on young age of Tsarnaev who was 19 at the time of the attacks.

Lawyers also highlighted his rough family life. The Tsarnaevs - ethnic Chechens - had lived in the former Soviet republic of Kyrgyzstan and the volatile Dagestan region of Russia, near Chechnya, before moving to the U.S. in 2002.

Teachers called Tsarnaev "sweet" and "hardworking" while other witnesses said his mother was obsessed with religion and his father had post-traumatic stress disorder, contributing to his actions.

Tamerlan, who was 26, died after the bombings in a shootout with police.