Former President Joe Biden called these "dark days" as he urged Americans to stay optimistic and not to 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 Congress and an autonomous judiciary. With the federal government facing its second-longest shutdown on record, Trump has used the lapse in funding to exercise new command over the government.

"Friends, I can`t sugar coat any of this. These are dark days" Biden said. He then predicted the country would "find our true compass again" and "emerge as we always have - stronger, wiser and more resilient, more just, so long as we keep the faith."

Biden listed examples of people who are standing their ground against threats from the current administration, citing the example of federal employees who resign in protest, and universities and comedians that have been targeted by Trump.

"The late night hosts continue to shine a light on free speech knowing their careers are on the line," he said.

Biden also shouted out elected Republican officials who vote or openly go against the Trump administration.

"America is not a fairy tale," he said. "For 250 years, it`s been a constant push and pull, an existential struggle between peril and possibility."

He finished the speech by telling people to "get back up."

The Democrat left office in January after serving one term in the White House. Biden dropped his bid for reelection after facing pressure following a disastrous debate against Trump and concerns about his age, health and mental fitness. Vice President Kamala Harris launched her bid right after, but lost to Trump last November.

In May, Biden`s post presidential office announced that he had been diagnosed with prostate cancer and that it had spread to his bones.

Prostate cancers are graded for aggressiveness using what is known as a Gleason score. The scores range from 6 to 10, with 8, 9 and 10 prostate cancers behaving more aggressively. Biden`s office said his score was 9.