Mohamed Mohamud, 23, was found guilty last year following a three-week trial over the 2010 plot, after he was snared in an FBI sting operation involving a fake bomb that he tried to detonate.
Prosecutors said yesterday that Mohamud actively participated in the plot that could have killed thousands attending the Christmas tree lighting ceremony on November 26, 2010, had the bomb been real.
But lawyers for Mohamud claimed he was tricked by undercover agents from the Federal Bureau of Investigation.
After meeting the agent, Mohamud said he had been thinking of conducting a "holy war" since the age of 15, and suggested the plot to bomb the traditional pre-Christmas ceremony in the Oregon capital Portland.
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FBI undercover operatives cautioned Mohamud several times about the seriousness of the plan, noting that there would be many children at the event.
But Mohamud responded that he was looking for a "huge mass that will... Be attacked in their own element with their families celebrating the holidays," according to court documents.
He was arrested shortly after trying to detonate the fake bomb as the Christmas tree ceremony got under way in Portland's Pioneer Courthouse Square.
Mohamud was convicted in January last year of attempting to use a weapon of mass destruction.
He apologised in court shortly before being sentenced by US District court Judge Garr M King, whose assistant Pamela Graham confirmed the 30-year jail term to AFP.