Dzokhar Tsarnaev, suspected of carrying out the 2013 Boston Marathon bombing, has gone on trial in the biggest domestic terrorism case in the United States since the Oklahoma City bombing nearly 20 years ago.

Tsarnaev, 21, appeared before the US District Court in Boston on Wednesday - and his lawyer immediately told the jury he was guilty of taking part in the bombing.

"It was him," defence attorney Judy Clarke said of Tsarnaev.

Laying out an argument clearly aimed at saving Tsarnaev from the death penalty, Clarke said the defence will not try to "sidestep" his guilt for the "senseless, horribly misguided acts carried out by two brothers".

"The evidence will not establish and we will not argue that Tamerlan put a gun to Dzhokhar's head or that he forced him to join in the plan, but you will hear evidence about the kind of influence that this older brother had," she said.

Tamerlan died in a shootout with police days after the bombings.

About two dozen victims, including some who lost limbs in the attack, took up the entire left side of the courtroom. Also in the group were Denise and Bill Richard, parents of eight-year-old Martin Richard, who died in the bombings.

Three people were killed and more than 260 were hurt when two pressure-cooker bombs exploded near the finish line seconds apart on April 15, 2013.

Prosecutors say Dzhokhar was an equal participant who acted of his own free will.

He faces 30 charges in the bombings and the shooting death days later of a police officer from the Massachusetts Institute of Technology.

Seventeen of the charges carry the possibility of the death penalty.

Source: Agencies