President Joe Biden on Friday marked three years since the deadly Jan. 6 attack on the U.S. Capitol with a warning to voters that former president Donald Trump - Biden's likely 2024 election opponent - is a threat to the country's standing as a free democracy.

"He's willing to sacrifice our democracy, put himself in power."

Trump contested his defeat in the 2020 election against Biden - a Democrat - and failed.

But he pushed false claims that his defeat was tainted by fraud, claims that fueled the violent mob of his supporters who stormed the halls of Congress on January 6, 2021 in an attempt to prevent the certification of the election results.

BIDEN: "An attempt to overturn a free and fair election by force and violence. A record 81 million people voted for my candidacy and to end his candidacy. Trump lost the popular vote by 7 million."

Speaking near a Revolutionary War memorial in Pennsylvania, Biden warned that Trump's candidacy embraced the political violence of Jan. 6.

BIDEN: So hear me clearly: I'll say what Donald Trump won't: Political violence is never, ever acceptable in the United State political system. Never ever. It has no place in a political democracy, none."

And noted that Trump and his supporters treated example of political violence with amusement.

"He laughed about it. What a sick... My god."

Trump has been criminally charged by a special counsel for conspiring to illegally subvert the results of the 2020 election. The Republican has denied wrongdoing and called the prosecution politically motivated.

Ahead of Biden's speech, the Trump campaign released an ad accusing Biden of being "the true destroyer of democracy."

It said he was trying to "justify his push to imprison his leading political rival and deprive Americans of their right to choose their next president through corrupt political lawfare."

The 2024 race is expected to be closely contested - and Biden aides see Pennsylvania, home to Biden's Scranton birthplace, as a must-win state.