The accused bomber Dzhokhar Tsarnaev stood in a Boston federal court and calmly answered in a thick accent "not guilty" to using a weapon of mass destruction and 29 other charges relating to the attack on the Boston Marathon as they were read out.

At about midday Wednesday, a prisoner transport vehicle escorted by a Humvee filled with armed men swept past onlookers into the court complex. A small gaggle of supporters chanted, "Justice for Dzhokhar" and "Give him his freedom back".

Wearing an orange prison jumpsuit Tsarnaev appeared in a courtroom filled with survivors of the attack, media and a few of his supporters, while hundreds of others filled overflow rooms.

Of the 30 charges Tsarnaev faces, 17 could bring the death penalty, although it is not yet clear if the prosecution will seek capital punishment.