"What we saw on Wednesday was only the climax of very worrying developments happening globally in recent years. It must be a wake-up call for all democracy advocates," EU foreign policy chief Josep Borrell said in a blog post.
"Everybody needs to understand that if we accept setbacks after setbacks, even if they seem minor, democracy and its values and institutions can eventually and irreversibly perish," said Borrell, who speaks on behalf of the 27 EU member states.
President Donald Trump is facing a renewed drive by Democrats to remove him from office after he incited supporters to storm the U.S. Capitol, based on the unsubstantiated claim that he lost the Nov. 3 election due to widespread voter fraud.
The resulting assault by rioters left five people dead.
"In case anyone had any doubts, the events in Washington also show that disinformation constitutes a real threat for democracies," Borrell said. "If some people believe that an election was fraudulent, because their leader has been once and again telling them, they will behave accordingly."