A top EU official on tech policy said on Monday that the attack on US Congress by sympathisers of President Donald Trump would be seen by history as social media's September 11.

"Just as 9/11 marked a paradigm shift for global security, 20 years later we are witnessing a before-and-after in the role of digital platforms in our democracy," EU commissioner Thierry Breton wrote in Politico.

The fallout from the attack has led to Twitter banning Trump outright from its platform, in a move that has been welcomed by some, but criticised, even by the president's opponents.

Breton said the incidents in Washington revealed "the fragility of our democracies - and the threat that under-regulated tech companies can pose to their survival". 

The former French finance minister expressed serious doubts about whether social media companies alone should have the power to block accounts of a US president.

"The fact that a CEO can pull the plug on (Trump's) loudspeaker without any checks and balances is perplexing," Breton wrote.

"It is not only confirmation of the power of these platforms, but it also displays deep weaknesses in the way our society is organised in the digital space," he added.

Breton also defended the EU's recent proposals to more closely regulate big tech, including the Digital Services Act that could see platforms face fines for failures to curb illegal content.