Former US President Joe Biden on Tuesday slammed the Trump administration for "taking a hatchet" to Social Security in his first major public speech since leaving office in January.

"We can't go on like this as a divided nation, as divided as we are," the 82-year-old said at the Advocates, Counselors and Representatives for the Disabled conference in Chicago. "It's never been this divided."

Speaking in a subdued tone, Biden criticized the attacks by President Donald Trump and his far-right billionaire ally Elon Musk on government welfare programs, inflicting significant damage in less than 100 days in office.

He said about 7,000 government employees had been laid off, resulting in serious disruptions such as the crash of vital official websites due to IT cutbacks – all consequences of Musk's rampage through government agencies.

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Biden said that Social Security was a "sacred promise," saying "people are now genuinely concerned ... for the first and only time in history, that Social Security benefits may be delayed or interrupted."

At the end of his speech, Biden slowly descended the podium steps and briefly spoke with attendees in the front row.

The appearance comes amid reports that many Democrats had hoped Biden would quietly retire from public life after his disastrous final year in office which ultimately led to Trump's re-election.

The Democrat said the urgency of the current crisis had moved him to speak, as any potential interruptions to Social Security threaten a "calamity for millions of families."

The White House mocked Biden's intervention, with Press Secretary Karoline Leavitt saying: "I'm shocked that he is speaking at nighttime. I thought his bedtime was much earlier than his speech tonight."