X, previously known as Twitter, has been banned in Brazil after failing to comply with a Supreme Court deadline to appoint a new legal representative.
Judge Alexandre de Moraes ordered the “immediate and complete suspension” of the platform until it meets all court requirements and pays existing fines.
The ban follows ongoing tensions since April, when the judge ordered the suspension of accounts spreading disinformation.
Reacting to the decision, X owner Elon Musk said: “Free speech is the bedrock of democracy and an unelected pseudo-judge in Brazil is destroying it for political purposes.
The social media network is said to be used by at least a 10th of the nation’s 200 million inhabitants.
By Saturday morning some users had reported access to the platform was no longer possible.
It closed its office in Brazil earlier this month, saying its representative had been threatened with arrest if she did not comply with orders it described as “censorship” – as well as illegal under Brazilian law.
Justice Moraes had ordered that X accounts accused of spreading disinformation – many supporters of the former right-wing president Jair Bolsonaro – must be blocked while they are under investigation.
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He said the company’s legal representatives would be held liable if any accounts were reactivated.
X has been threatened with fines for refusing to comply with this order, with the company and Mr Musk joining critics in Brazil in accusing the judge as being left-wing.
It is the latest in a series of rows involving the tech billionaire – who has clashed with the EU over the regulation of X and earlier this month became embroiled in a war of words with UK Prime Minister Sir Keir Starmer.
The head of Brazil’s telecommunications agency, which has been tasked with suspending the platform, said he is “proceeding with the compliance” to do so, according to Reuters news agency.
Justice Moraes has given companies such as Apple and Google a five-day deadline to remove X from its application stores and block its use on iOS and Android systems.
He added that people or businesses using means such as VPNs (virtual private network) to access the platform could be fined R$50,000 (£6,700).
According to the judge’s order, a ban will be in effect until X names a new legal representative in the country and pays fines for violating Brazilian law.
In a previous post from one of its official accounts, X had said it would not comply with the demands.
“Soon, we expect Judge Alexandre de Moraes will order X to be shut down in Brazil – simply because we would not comply with his illegal orders to censor his political opponents,” the post said.
“The fundamental issue at stake here is that Judge de Moraes demands we break Brazil’s own laws. We simply won’t do that.”