"It WAS him," said defence attorney Judy Clarke, one of America's foremost death-penalty specialists.
Three people were killed and more than 260 hurt when two shrapnel-packed pressure-cooker bombs exploded near the finish line on April 15, 2013. Tsarnaev, then 19, is accused of carrying out the attacks with 26-year-old brother Tamerlan, who was killed in a shootout and getaway attempt days later.
In a strategy aimed at saving Tsarnaev from a death sentence, she argued that he had fallen under the influence of his older brother.
"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," Clarke said, "but you will hear evidence about the kind of influence that this older brother had."
Federal prosecutors used their opening statements — along with chilling video and testimony from witnesses — to paint Tsarnaev as a cold-blooded killer and sketch a grisly picture of torn limbs, screams and the smell of sulfur and burned hair in the streets.
Tsarnaev planted a bomb designed to "tear people apart and create a bloody spectacle," then hung out with his college buddies as if he didn't have a care in the world, federal prosecutor William Weinreb said.
"He believed that he was a soldier in a holy war against Americans," Weinreb said. "He also believed that by winning that victory, he had taken a step toward reaching paradise. That was his motive for committing these crimes."
A shaggy-haired, goateed Tsarnaev slouched in his seat and showed no reaction as Weinreb spoke, not even when the prosecutor described how Tsarnaev ran over his brother with a stolen Mercedes and dragged the body 50 feet (15 metres) during the getaway.
About two dozen victims took up one entire side of the courtroom. Several hung their heads and appeared to fight back tears.
Prosecutors contend the brothers — ethnic Chechens who arrived from Russia more than a decade ago — were driven by anger over U.S. wars in Muslim lands.
Because of a wealth of evidence against the younger brother — including a video of him leaving a backpack at the scene, and incriminating graffiti scrawled on the boat where he was captured — legal experts have said there is little chance of escaping conviction during the guilt-or-innocence phase of the trial.
Instead, they said, Tsarnaev's lawyers will concentrate on saving his life by arguing that the radicalized Tamerlan was the driving force in the plot.
Clarke, in an opening statement that took less than 20 minutes, called the bombings "senseless, horribly misguided acts." But she asked the jurors to "hold your hearts and minds open" until the penalty phase, when the panel will decide whether Tsarnaev should be executed or get life in prison.
In an apparent attempt to show that the attack was calculated for maximum carnage, prosecutors called as the trial's first witness Thomas Grilk, executive director of the Boston Athletic Association, which oversees the 118-year-old marathon.
Grilk said that up to a half-million spectators turn out for the race each year, with people lining the streets in cities and towns along the 26.2-mile (42-kilometre) course and huge crowds near the finish line in Boston.
The prosecutor also described how 8-year-old Martin Richard stood on a metal barrier with other children so he could get a good view of the runners. The boy bled to death on the sidewalk as his mother looked on helplessly, Weinreb told the jury, with the youngster's parents in the courtroom.
Right up until the moment the jury filed into the courtroom, Tsarnaev's lawyers fought to have the trial moved out of Massachusetts, arguing that the emotional impact of the bombings ran too deep and too many people had personal connections to the case. But U.S. District Judge George O'Toole Jr. and a federal appeals court rejected the requests.
The panel of 10 women and eight men was chosen Tuesday after two long months of jury selection, interrupted repeatedly by snowstorms and the requests to move the trial, which is expected to last three to four months.
The case is the most closely watched terrorism trial in the U.S. since the Oklahoma City bombing case in the mid-1990s.