BOSTON (AP) — His life on the line, Dzhokhar Tsarnaev went on trial Wednesday in the Boston Marathon bombing, with prosecutors saying he used a backpack to plant a bomb designed to "tear people apart and create a bloody spectacle."

A shaggy-haired Tsarnaev, 21, stared straight ahead and did not look at either the jury or Assistant U.S. Attorney William Weinreb as the prosecutor launched into his opening statement in the nation's most closely watched terrorism trial since the Oklahoma City bombing more than 20 years ago.

About two dozen victims of the Boston attack took up the entire left-hand side the courtroom.

Weinreb said Tsarnaev carried a bomb in a backpack, and it was "the type of bombs favored by terrorists because it's designed to tear people apart and create a bloody spectacle."

Sketching out the horrific scene on the streets after the two pressure-cooker bombs exploded, Weinreb said: "The air was filled with the smell of burning sulfur and people's screams."

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

Just before the jury was brought in, the judge rejected a fourth request from Tsarnaev's lawyers to move the trial out of Boston.

Two dramatically different portraits of the former college student are expected to emerge during the trial.

Was he a submissive, adoring younger brother who only followed directions given by his older, radicalized brother? Or was he a willing, active participant in the attacks?

Among the victims in the courtroom was Heather Abbott, who lost a leg in the attack. None of the victims came in on crutches or in wheelchairs; all appeared to walk under their own power.

Also in the group were Denise and Bill Richard, the parents of 8-year-old Martin Richard, who died in the bombings.

Tsarnaev's lawyers have made it clear they will try to show that at the time of the attack, Tsarnaev, then 19, looked up to his older brother, Tamerlan, 26, and was heavily influenced by him. They plan to portray Tamerlan as the mastermind of the attack. He died in a shootout with police days after the bombings.

But prosecutors say Dzhokhar was an equal participant who acted of his own free will. They 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.

Tsarnaev 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.

Security was expected to be extremely tight. During jury selection, dozens of police officers and federal security officers were stationed inside and outside the courthouse, armed Coast Guard boats patrolled Boston Harbor, and a side street leading to the courthouse was blocked.

Tsarnaev's lawyers fought right up until the last minute to have the trial moved outside of Massachusetts, arguing that the emotional impact of the bombings ran too deep and too many people had personal connections to the case. Their requests were rejected by Judge George O'Toole Jr. and a federal appeals court.

A panel of 10 women and eight men was chosen Tuesday to hear the case after two long months, interrupted repeatedly by snowstorms and the requests to move the trial.

The trial will be split into two phases — one to decide guilt or innocence, the other to determine punishment. If Tsarnaev is convicted, the jury will decide whether he gets life in prison or death.

The trial is expected to last three to four months.

The list of witnesses remains sealed, but among those expected to testify are first responders who treated the wounded, marathon spectators and victims who were badly injured in the explosions.

Attorney Judy Clarke, one of the nation's foremost death penalty specialists, was expected to deliver the opening statement for Tsarnaev.

Clarke has saved a string of high-profile clients from the death penalty, including Atlanta Olympics bomber Eric Rudolph; Unabomber Ted Kaczynski; and Jared Loughner, the man who killed six people and gravely wounded then-Rep. Gabrielle Giffords in a 2011 shooting in Tucson, Arizona.