Lee Rigby was knocked down by a car and then nearly decapitated as Michael Adebolajo, 29, and Michael Adebowale, 22, hacked him to death with a meat cleaver and knives outside his barracks on May 22.
Jurors took just over an hour to reach their verdict. They found the pair not guilty of attempted murder of a police officer.
Adebolajo kissed his copy of the Koran as he was taken down to the cells at the Old Bailey court in London.
They considered themselves "soldiers of Islam", the court heard, and believed they were avenging the deaths of Muslims.
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Their defence lawyers told the court they had sought martyrdom by challenging armed police in the hope of being killed. Adebolajo was shot as he charged at an armed officer, while Adebowale was shot as he raised a rusty gun that the pair had brought to the scene.
The pair, both from Nigerian Christian families and born and raised in Britain, converted to Islam.
Adebolajo was known to the security services, having been arrested in Kenya in 2010 and deported.