Michael Adebolajo, 28, and Michael Adebowale, 22 - both Britons of Nigerian descent - drove at soldier Lee Rigby before attempting to decapitate him near his Army barracks in Woolwich, south-east London, on May 22, jurors at the Old Bailey court.
Setting out the case, prosecutor Richard Whittam told the jury that the two men had committed a "cowardly and callous" attack on Rigby and had "mutilated his body with a meat cleaver and knives".
The accused were flanked by heavy security as they appeared in a glass-lined dock in the courtroom.
"They both attacked the motionless body of Lee Rigby. He was repeatedly stabbed and Michael Adebolajo made a serious and almost successful attempt to decapitate him with multiple blows to his neck made with the meat cleaver," Whittam said.
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"At the same time as Michael Adebolajo used the meat cleaver, Michael Adebowale was using a knife to stab and cut at Lee Rigby's body," he said.
Rigby's family members were in the court to hear the opening of the case.
Brown paper was taped over part of the dock where the defendants sat so that they were shielded from the view of the relatives and friends of the victim.
The case was due to open last week but had been delayed by legal argument.
"The jurors who try this case must do so impartially, and thus based on only the evidence that those jurors are going to hear in this courtroom and absolutely nothing else," he said.