US President Joe Biden joked about Donald Trump and his own age at an annual media dinner on Saturday -- before unloading deadly serious criticisms of his rival in November's election.

"One candidate's too old and mentally unfit to be president," the 81-year-old Democrat quipped at the Gridiron Club in Washington. "The other guy's me."

Democrat Biden was making his first speech as president at the annual white tie gala for the US media and political elite, an event that Republican former president Trump addressed in 2018.

Biden is trailing in a number of polls and faces voters concerns about his age, which he has tried to address by highlighting 77-year-old Trump's recent verbal slip-ups.

In his remarks, Biden took a swipe at Republicans in Congress who have launched an impeachment inquiry into his son's business dealings, saying they would "rather fail at impeachment than succeed at anything else."

He added that Irish Prime Minister Leo Varadkar, sitting at the head table with Biden on the eve of St. Patrick's Day "took one look at Congress and he asked for another Guinness."

Varadkar and Biden both pushed during a meeting at the White House on Friday for Republicans in Congress to stop blocking military aid for Ukraine to fight Russia's invasion.

But Biden then returned to Trump, saying that the Democrats' election campaign would show how they rebuilt the US economy after the Covid-19 pandemic "without encouraging the American people to inject bleach."

He was referring to an incident when Trump, as president, asked a top medical advisor whether virus victims could be injected with disinfectant to cure them.

"Look, I wish these were jokes, but they're not," added Biden.

"Democracy and freedom are literally under attack. Putin's on the march in Europe. My predecessor bows down to him and says, 'Do whatever the hell you want.'"

Noting that Estonia's Prime Minister Kaja Kallas, a strong critic of Russia, was also at his table, he added "We will not bow down, they will not bow down and I will not bow down."

Biden added that Trump's false claims to have won the 2020 election, and the January 6 2021 Capitol assault by pro-Trump rioters, showed there was "poison coursing through the veins of our democracy."

He also backed journalists whom Trump has repeatedly attacked, adding: "You are not the enemy of the people. You are a pillar of any free society."

In his own appearance at the Gridiron Club six years ago, Trump did trade unusually playful digs with the Washington press corps and also joked about North Korea and his own leadership style.

The Gridiron dinner -- held behind closed doors with no photos allowed -- sees Washington's elite unwind for a night of self-deprecatory humor which includes costumed members performing a song.