AFP/ Boston

Boston bomber Dzhokhar Tsarnaev returned to court Tuesday for the start of his sentencing trial where an American jury will condemn the 21-year-old to death or life without parole.

The immigrant of Chechen descent was convicted earlier this month on all 30 counts related to the 2013 marathon bombings, the murder of a police officer, a carjacking and a shootout while on the run.

The federal court room was packed for the first day of the penalty phase, which began with instructions from the judge to the jury, the same 12-member panel that convicted Tsarnaev on April 8.

Outside the building, a dozen protesters demonstrated against the death penalty, holding up banners, one of which read: "Why do we kill people who kill people to show that killing people is wrong?"

This second phase of the trial begins a day after 27,000 people took part in this year's Boston Marathon, which is still reeling from the memory of the attacks, the deadliest in the United States since 9/11.

It also comes as prominent survivors have opposed the death penalty for Tsarnaev, a teenage student at the time, who with his elder brother killed three people and wounded 264 others in the bombings.

The convicted killer walked into the court room, staring at his feet and dressed in a grey T-shirt, black blazer and dark pants.

The penalty phase, which is expected to last three to four weeks at the federal court in the northeastern US city, will see both prosecutors and defense attorneys call witnesses.

It is unclear whether Tsarnaev, who has been a silent if fidgety presence in court, or any of his relatives will take the stand.

- Aggravating factors -

His parents now live in Russia, although his two sisters and Tamerlan's widow, a US-born Muslim convert, live in the United States.

Prosecutors will try to convince the 12 jurors that there are enough aggravating factors -- including premeditation, the number of victims and a lack of remorse -- to warrant capital punishment.

The defense will argue their client should be sent to prison, portraying him as a confused 19-year-old, frightened of his radicalized brother Tamerlan, who was shot dead by police while on the run.

Seventeen of his 30 convictions carry the death penalty under federal law.

Jurors were selected in part for their openness to impose the death penalty, controversial in a state that has executed no one since 1947 and where Catholic bishops oppose capital punishment.

On Friday, Bill and Denise Richard, whose eight-year-old son Martin was killed and then six-year-old daughter Jane lost a leg, said pursuit of the death penalty could entail years of appeals and "prolong reliving the most painful day of our lives."

Married couple Jessica Kensky and Patrick Downes, who each lost limbs, have also called for life without parole or appeal.

Chief defense lawyer Judy Clarke is one of America's leading experts on capital punishment and has saved a string of high-profile clients from death row.

- Must be unanimous -

Statistics are on Tsarnaev's side.

Since the federal death penalty was reinstated in 1984, only 79 people have been sentenced to die on federal offenses and of those only three have been executed, says the Death Penalty Information Center.

They were Oklahoma bomber Timothy McVeigh and drug trafficker Juan Garza in 2001, and Gulf War veteran Louis Jones in 2003 for the kidnap, rape and murder of a 19-year-old female Army recruit.

Three other defendants received death verdicts, which were turned into life sentences after new trials were granted.

A nationwide poll carried out last month showed that support for the death penalty has fallen to its lowest level in 40 years.

Yet the Pew Research Center still found that 63 percent of Americans believe the death penalty is morally justified for a crime like murder.

The jury's sentencing decision has to be unanimous. If just one juror believes in extenuating circumstances, then Tsarnaev will be sentenced to life in prison.

They must also be unanimous on whether the different aggravating factors are enough to sentence him to death.