Gruesome details of injuries sustained by victims of the Boston Marathon bombings have been recounted to jurors, as lawyers for the Muslim student on trial for the attack admitted he carried out the bloody assault.

Nearly two years after what was the worst attack on US soil since 9/11, Dzhokhar Tsarnaev’s trial got under way on Wednesday in a federal court packed with victims, survivors and the media.

Prosecutors said Tsarnaev, 21, had carried out the attacks to avenge the deaths of Muslims overseas after learning how to build pressure-cooker bombs through Al-Qaeda propaganda.

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The Kyrgyzstan-born US citizen faces the death penalty if convicted of the bombings which killed three people and wounded 264, on April 15, 2013.

In a dramatic opening statement by Tsarnaev’s defence team, lawyer Judy Clarke admitted her client had planted the bombs, telling jurors bluntly, “It was him.”

“We do not and will not at any point sidestep or attempt to sidestep Dzhokhar’s responsibility for his actions,” she said.

She said the defence would agree with the government about much of the evidence and instead appealed on the jury to keep their hearts and minds open for the second half of the trial focused on the sentencing.

Government prosecutors meanwhile painted the picture of a cold, callous killer who calmly shopped for milk just 20 minutes later, as paramedics battled in vain to save a mortally wounded eight-year-old boy.

Assistant US attorney William Weinreb used much of his 50-minute address to emphasise the horror unleashed when Tsarnaev and his 26-year-old brother Tamerlan allegedly planted two bombs near the race finish line.

As the jury were shown pictures of victims Krystle Marie Campbell, Lingzi Lu and Martin Richard in happier days, Weinreb described in graphic detail the injuries that saw them bleed to death on the sidewalk.

Boston marathon bombing victims Krystle Campbell, Lu Lingzi and Martin Richard. Photo: AAP

Martin was so small that the bomb damaged his entire body, exposing his ribs and organs, eviscerating his bowels and burning his skin. He lost so much blood that he had practically none left, the prosecutor said.

“The purpose of this bomb was to shred flesh, shatter bones, set people on fire,” Weinreb told the court.

“The purpose was to kill and maim as many people as possible.”

Tsarnaev showed no emotion as the details of his alleged crimes were read out.

Weinreb said that Tsarnaev may have lived outwardly the life of an ordinary University of Massachusetts student but “he had a side to him that he kept hidden” and was reading terrorist literature as early as 2011.

It was that literature that convinced him to kill Americans to punish them for killing Muslims overseas and to earn him a place in paradise, he said.

Tsarnaev, who became a US citizen in 2012, has pleaded not guilty to 30 charges over the attacks, the subsequent killing of a police officer, a car jacking and shootout with police while on the run.

Tamerlan was killed in the shootout, and run over by his brother who eventually surrendered on April 19 after being discovered in a boat in a suburban backyard.

Seventeen of the 30 charges against him carry the possibility of a death penalty under federal law, but Massachusetts abolished the death penalty in 1984 and no one has been executed in the state since 1947.