By Scott Malone and Elizabeth Barber

BOSTON (Reuters) - A lawyer for the accused Boston Marathon bomber said at the start of his trial that their client bore responsibility for the attacks that killed three people and injured 264 with a blunt admission: "It was him."

But Dzhokhar Tsarnaev was a secondary player in the April 15, 2013 bombings at the famous race and the fatal shooting days later of a police officer, defense attorney Judith Clarke said in her opening argument in U.S. District Court in Boston. She indicated that the 21-year-old's older brother, Tamerlan, was the prime mover.

The defense strategy, which did not include a move to change Tsarnaev's not-guilty plea, appeared aimed at sparing Tsarnaev from the death penalty. If he is convicted, the jury will decide whether he is executed or gets life in prison without possibility of parole.

A prosecutor, William Weinreb, told jurors how Tsarnaev and his brother, both ethnic Chechens, carefully selected the places where they left the bombs in an effort to punish the United States for military actions in Muslim-dominated countries.

Assistant U.S. Attorney Weinreb anticipated the defense's strategy of casting blame on 26-year-old Tamerlan Tsarnaev, who was killed when his younger brother inadvertently ran him over after a gunbattle with police a few days after the bombing.

"The focus is going to be on the defendant," the prosecutor said. "That's because it's his day in court. He's the one the government has to prove guilty, not his brother."

But Clarke said although Dzhokhar Tsarnaev, then 19, set off a homemade pressure-cooker bomb at the marathon's crowded finish line and three days later participated in the fatal shooting of the police officer did not tell the whole story.

"If the only question was whether that was Dzhokhar Tsarnaev ... it would be very easy for you,"

Clarke said her client played a secondary role in a scheme hatched and driven by his older brother.

"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. He 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, which would follow providing Dzhokhar is found guilty.

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

'SOLDIER IN A HOLY WAR'

In his argument, Weinreb said that Dzhokhar had read radical online magazines, where he learned how to build the shrapnel-filled bombs that tore the legs off of about 16 people at the race's crowded finish line.

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

The prosecutor also stressed the brutal nature of the injuries caused by the pressure-cooker bombs.

"The purpose of that kind of bomb is to shred flesh, shatter bone, set people on fire and cause people to suffer painful deaths," said Weinreb. He described the painful final moments of the three who died: restaurant manager Krystle Campbell, 29; graduate student Lingzi Lu, 23, and Martin Richard, 8.

He noted that Tsarnaev left the backpack he carried the bomb in behind a group of child spectators, including Martin.

"He pretended to be a spectator but he had murder in his heart," Weinreb said.

The prosecutor also described in detail how Dzhokhar Tsarnaev killed his brother following a gunbattle with police in the Boston suburb of Watertown, while the two were trying to flee.

"The defendant rolled right over his brother and dragged his body about 50 feet (15 meters) down the street," Weinreb said. The defendant had been trying to run down the police officers who were attempting to arrest the pair.

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 Richard's parents, sat quietly in court during the proceedings.

The jury saw several videos of the bombs going off and of badly injured people screaming on the ground, surrounded by bits of metal shrapnel and thick smoke.

"I was blown through the air. There was a deafening explosion," said Colton Kilgore, who had traveled from Asheville, North Carolina, to see his mother-in-law race. "As I was sitting up my brain was in this haze and I couldn't hear out of my left ear and there was screaming and I realized ... it must have been a bomb."

(Reporting by Scott Malone; Editing by Bill Trott, Jonathan Oatis, Grant McCool)