BOSTON (AFP) - The long-awaited trial of Boston Marathon bombing suspect Dzhokhar Tsarnaev began on Monday with jury selection, 20 months after the devastating attack that killed three at the city's signature race.

Tsarnaev, a 21-year-old Muslim of Chechen origin, faces the death penalty if convicted in federal court for what was the worst act of terror on US soil since the Sept 11 attacks in 2001.

Dressed in a dark pullover and light pants and with a shock of tangled hair and small beard, Tsarnaev appeared opposite a first group of 200 to 250 potential jurors as US District Judge George O'Toole briefly reviewed the charges against him.

"What you need is a commitment to justice," Judge O'Toole advised the jurors.

Tsarnaev - flanked by his defence team - has pleaded not guilty to the 30 charges, including the use of a weapon of mass destruction leading to death, and carrying out an attack in a public place.

With his older brother, Tamerlan, Tsarnaev is alleged to have set the bombs, which exploded 12 seconds apart on April 15, 2013, ripping through crowds gathered near the finish line as the race was ending.

More than 260 people were wounded. Several people lost limbs when the pressure-cooker bombs - packed with nails and other shrapnel - exploded.

Tamerlan was killed in a confrontation with police days after the attacks.

After a manhunt that included thousands of police, Tsarnaev was arrested a few hours after the death of his brother. He was seriously injured and found hiding in a boat in a Boston suburb.

A 12-member jury, with six alternates, will be selected from a pool of about 1,200. Each potential juror must fill out a questionnaire; those answers will eventually be used by attorneys for juror elimination.

Only one survivor of the attack was seen in court Monday, but more are expected to attend once the proceedings begin in earnest.

For the death penalty case in a city where memories of the attack are still fresh and raw, jury selection is expected to be fastidious.

If Tsarnaev is found guilty, the jury must unanimously decide between capital punishment and life imprisonment.

Defence attorneys had tried in vain to have the trial moved to another city, arguing it would be impossible to assemble an impartial jury in Boston.

Judge O'Toole said opening arguments would begin on Jan 26, and the trial was expected to last three to four months.

The prosecution will be one of the most important for the government since the Timothy McVeigh bombing trial in 1997 that ended in a death sentence.

Tsarnaev's defence team includes Ms Judy Clarke, an expert in death penalty cases who has helped spare her clients from capital punishment numerous times.

Tsarnaev has been held in near-solitary confinement in federal prison outside Boston. He has made two appearances since his arrest, once to plead not guilty in 2013 and again at a pre-trial hearing last month.

According to the indictment, the government believes the Tsarnaev brothers acted alone. The government portrays them as radicalised lone wolves, who learned how to make bombs from an Al-Qaeda publication online.

A message inside the boat where Tsarnaev was found after the manhunt offers a glimpse into the possible motive behind the crime: "The US government is killing our innocent civilians... We Muslims are one body... Stop killing our innocent people and we will stop," it read.

Tsarnaev's family, who live in Russia, were not present and have kept a low profile since an emotional press conference just after the bombing.

His mother, Ms Zubeidat Tsarnaeva, accused the US security services of needlessly killing Tamerlan while trying to capture him.

Both parents have stressed their sons' innocence and lamented ever going to the United States.