National courts in EU states can order Facebook to delete content "worldwide", Europe's top tribunal has ruled, in what the US social media giant called an attack on free speech.

If content was deemed "illegal" by a national court, then Facebook could be ordered to "remove information covered by the injunction or to block access to that information worldwide", the European Court of Justice (ECJ) said in Luxembourg on Thursday (3 October).

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That meant, for instance, that if a court in Austria ruled that a news article about a politician was slanderous, then it could force the US giant to "remove" or "block" it not just in all the 28 EU member states but for all of its more than 2bn users world wide.

It would have to block original material as well as "content which is equivalent", the ECJ added.

But Facebook would not itself be liable for damages if it "expeditiously" pulled down the illicit posts, the court also said, in its reading of a 20-year old EU "e-commerce" law.

Thursday's ECJ ruling came after an Austrian Green politician, Eva Glawischnig-Piesczek, sued Facebook over a story called "Greens: Minimum income for refugees should stay", which said she was a "traitor ... corrupt ... [and] fascist", published by the oe24.at news website.

Austrian courts had pulled it off the internet, but a Facebook user in Ireland had posted it on their page, creating a universally visible thumbnail image of the story, and Glawischnig-Piesczek wanted Facebook Ireland to also take steps.

The ECJ ruling could have "a chilling effect on freedom of expression" and undermined the "the long-standing principle that one country does not have the right to impose its laws on speech on another country," Facebook CEO Mark Zuckerberg said after the verdict came out.

"It's a very troubling precedent ... This is something I expect us and other companies will be litigating," he also said in a live-streamed Q&A on his own website.

The EU's extra-territorial claims come after self-regulation by social media giants led to international abuse, such as Russia's meddling in the 2016 US election.

It also led to proliferation of far-right hate speech and radical Islamist content, posing potential security risks.

The ECJ ruling was "saying one must create and strengthen instruments for the enforcement of personal rights", Maria Windhager, Glawischnig-Piesczek's lawyer, told Austrian broadcaster ORF.

The decision would increase "awareness that we can successfully defend ourselves against" defamation and "strengthens all of us in the fight against online hate speech," Windhager added.

But each of the 28 EU states has a different libel regime, so that a post which violated German privacy laws, for instance, might be legal in France, complicating enforcement.

Max Schrems, a privacy campaigner, said the verdict could be used to identify Facebook users who had wanted to remain anonymous in members-only groups.

The ruling on taking down similar or "equivalent" content could also get bogged down in semantic and cultural debate on synonyms for words such as "traitor" or "corrupt" in Europe's more than 20 national languages.

The US firm would be obliged to weigh up "the differences in the wording of that equivalent content, compared with the wording" of the libellous story, the ECJ said.

But it did not "set out very clear definitions on what 'identical' and 'equivalent' means", Facebook complained.

Social media firms could "have recourse to automated search tools and technologies" to filter non-compliant content, the EU verdict also noted.

But that posed broader ethical questions on AI self-censorship, a UK digital rights group, called Article 19, warned.

And with some EU courts accused of corruption and political bias, the ECJ decision had another sinister side, it added, echoing Zuckerberg of free speech.

The EU itself has launched sanctions procures against Hungary and Poland after ruling parties there created judiciaries which could be used to gag political opponents.

"This [the ECJ ruling] could be open to abuse, particularly by regimes with weak human rights records," Article 19 said.