US President Donald Trump's administration imposed visa bans on Tuesday on five people, including French former EU commissioner Thierry Breton.
It accuses them of working to censor freedom of speech or unfairly target US tech giants with burdensome regulation.
The bans mark a fresh escalation against Europe, a region the US argues is fast becoming irrelevant due to its weak defences, inability to tackle immigration, needless red tape and "censorship" of nationalist voices to keep them from power.
In Brussels, Paris and Berlin, senior officials condemned the US bans and defended Europe's right to legislate on how foreign companies operate locally.
A European Commission spokesperson said it "strongly condemns the US decision," adding: "Freedom of expression is a fundamental right in Europe and a shared core value with the United States across the democratic world."
The spokesperson said the EU would seek answers from the United States but said it could "respond swiftly and decisively" against the "unjustified measures".
French President Emmanuel Macron, who has been travelling across France to warn about the dangers that disinformation poses to democracy, said he had spoken with Breton and thanked him for his work.
"We will not give up, and we will protect Europe's independence and the freedom of Europeans," Macron said on X.
Breton, a former French finance minister and the European commissioner for the internal market from 2019 to 2024, was one of the architects of the EU's Digital Services Act.
A landmark piece of legislation, the DSA aims to make the internet safer by compelling tech giants to do more to tackle illegal content, including hate speech and child sexual abuse material.
But the DSA has riled the US administration, which accuses the EU of placing "undue" restrictions on freedom of expression in its efforts to combat hateful speech, misinformation and disinformation.
It also argues that the DSA unfairly targets US tech giants and US citizens.
US officials were particularly upset earlier this month when the EU sanctioned Elon Musk's X platform, fining it 120 million euros ($A211 million) for breaching online content rules.
The bans also targeted Imran Ahmed, the British CEO of the US-based Center for Countering Digital Hate; Anna-Lena von Hodenberg and Josephine Ballon of the German non-profit HateAid; and Clare Melford, co-founder of the Global Disinformation Index, according to US Under Secretary for Public Diplomacy Sarah Rogers.
Germany's justice ministry said the two German activists had the government's "support and solidarity" and the visa bans on them were unacceptable, adding that HateAid supported people affected by unlawful digital hate speech.
"Anyone who describes this as censorship is misrepresenting our constitutional system," it said in a statement.
"The rules by which we want to live in the digital space in Germany and in Europe are not decided in Washington."
The United Kingdom said it was committed to upholding the right to free speech.
"While every country has the right to set its own visa rules, we support the laws and institutions which are working to keep the internet free from the most harmful content," a UK government spokesperson said in a statement.
A Global Disinformation Index spokesperson called the visa bans "an authoritarian attack on free speech and an egregious act of government censorship".
"The Trump Administration is, once again, using the full weight of the federal government to intimidate, censor, and silence voices they disagree with," they said.
"Their actions today are immoral, unlawful, and un-American."
