(Reuters) - Attorneys for the accused Boston Marathon bomber opened his trial on Wednesday with a stunning statement about their client, charged with killing three people and injuring 264: "It was him."

Defense and prosecution attorneys gave opening statements on the first day of the trial in Boston that will determine whether Dzhokhar Tsarnaev is found guilty of the largest mass-casualty attack on U.S. soil since Sept. 11, 2001. If he is convicted, the jury will determine whether he is sentenced to death.

Defense attorneys sought to skip straight past the question of guilt and turn the focus to the relative responsibility of the 21-year-old defendant and that of his 26-year-old brother.

It was Tamerlan Tsarnaev - who Dzhokhar killed days after the April 15, 2013 attack by inadvertently running him over with a car as they were trying to flee police - who was the mastermind, defense attorney Judith Clarke told the jury in her opening statement.

"It was Tamerlan Tsarnaev who self-radicalized. It was Dzhokhar who followed him," Clarke said. "The evidence will show that Tamerlan planned and orchestrated and enlisted his brother into this series of horrific acts."

The approach set up an immediate conflict with U.S. District Judge George O'Toole, who had ruled shortly before opening statements that the question of the relative culpability of the two brothers was best left to the trial's second phase, once Dzhokhar's guilt has been established.

"Some evidence of the brother's interactions will be inevitable," O'Toole allowed in brief remarks before the trial opened. But he interrupted Clarke multiple times to warn her against going too deep into family history.

Earlier, Assistant U.S. Attorney William Weinreb described how the defendant, and his brother carefully selected the places where they left the bombs in an effort to punish the United States for military actions in Muslim-dominated countries.

"He believed that he was a soldier in a holy war against Americans," Weinreb said of Dzhokhar.

Tsarnaev, who sat quietly in court wearing a white shirt and sport coat but no tie, has pleaded not guilty to all charges in a 30-count indictment.

A dozen or so people injured in the attack and family members, including dancer Heather Abbott and Marc Fucarile, both of whom lost legs in the blasts, and the parents of the youngest fatality, 8-year-old Martin Richard, sat quietly in court during the proceedings.

(Story corrects year of Sept. 11 attack to 2001, 2nd paragraph)

(Editing by Eric Beech, Bill Trott and Jonathan Oatis)