The European Union's top officials expressed relief on Wednesday that they would again have a friend in the White House as Joe Biden replaces Donald Trump.

Charles Michel, president of the European Council, said the moment should be seized to form a new "founding pact" between democratic powers.

He was joined in the European Parliament by Ursula von der Leyen, president of the European Commission, to address MEPs hours ahead of Biden's inauguration.

Trump's Washington has been at loggerheads with Brussels over trade, subsidies for planemakers and European attempts to tax and regulate American tax giants.

European capitals have also been outraged by Trump's decision to quit the Paris climate accord and the Iran nuclear deal -- and EU leaders hope Biden will return to them.

But some in Brussels also see parallels between Trump's rise and the success of populist authoritarianism in parts of Europe, and in this a threat to transatlantic democracy,    

"This time-honoured ceremony on the steps of the US Capitol will be a demonstration of the resilience of American democracy," von der Leyen said. 

"And the resounding proof that, once again, after four long years, Europe has a friend in the White House."

But Brussels' top official warned the fact that millions of Americans had voted for Trump and that some of them had rioted on his behalf on January 6 should serve as a warning.

"President Trump may be history in a few hours, but his supporters remain. More than 70 million Americans voted for Trump in the election," she said. 

"A few hundred of them stormed the Capitol in Washington, the heart of American democracy, just a few days ago.

"And this really is what it looks like when words are put into deeds, when hate speech and fake news become a real danger to democracy," she said

Absurd fantasies

"Despite established democracy being embedded here, we cannot assume that we are immune to these phenomena."

Von der Leyen compared the US riot to a recent attempt by German activists to storm Berlin's Reichstag building to protest coronavirus lockdown measures.

And she cited the case of Jo Cox, a British MP who was murdered by a far-right activist weeks before the UK voted to leave the European Union. 

"Of course, there are differences with the storming of the Capitol, but there are also people here who feel wronged and angry", she warned. 

Europeans should beware of "people who adhere to wildly rampant conspiracy theories, an often confused mixture of completely absurd fantasies", she said.

Michel, who hoped to host Biden at an EU-US summit in parallel with a NATO meeting in Brussels early in the new president's term, struck a more upbeat tone.

"Law, order and democracy have prevailed over this disgraceful attempt to overturn the election. American democracy has proven its strength and resilience," he said.

"Let's build a new founding pact for a stronger Europe, for a stronger America and for a better world."