Former US President Joe Biden on Sunday warned that the United States is going through “dark days” as he urged Americans to remain hopeful and defend democratic values.

Speaking publicly for the first time since finishing radiation therapy for aggressive prostate cancer, Biden said the country is witnessing attacks on free speech and the constitutional limits of presidential power under President Donald Trump.

He made the remarks while receiving the Lifetime Achievement Award from the Edward M. Kennedy Institute in Boston.

Biden said the United States has long stood as a “beacon” of an idea stronger than any army or dictator, but stressed that it depends on a presidency with limited authority, a functioning Congress and an independent judiciary.

The federal government is currently in its second-longest shutdown, and Trump has used the situation to expand his control over government operations, he said.

“I can’t sugarcoat any of this. These are dark days,” Biden told the audience. He added that the US will “find our true compass again” and become stronger and more just if Americans continue to keep faith in democracy.

He praised career officials who resigned in protest, universities and comedians facing pressure, and Republican lawmakers who challenged the administration.

“For 250 years, America has been a constant struggle between danger and opportunity,” he said, urging people to “get back up.”

Biden, 82, left office in January after one term and later dropped his re-election bid due to age, health concerns and a poor debate performance against Trump. His office announced in May that his prostate cancer had spread to his bones, with a high Gleason score indicating aggressive disease.