Former US president Joe Biden called these "dark days" as he urged Americans to stay optimistic, not check out in response to what he says are attacks on free speech and tests on the limits of executive power by President Donald Trump.

"Since its founding, America served as a beacon for the most powerful idea ever in government in the history of the world," Biden said.

"The idea is stronger than any army. We're more powerful than any dictator."

Biden, 82, speaking publicly for the first time since completing a round of radiation therapy for an aggressive form of prostate cancer, addressed an audience in Boston on Sunday night after receiving the Lifetime Achievement Award from the Edward M Kennedy Institute.

He said America depends on a presidency with limited power, a functioning Cong

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