BRASILIA (Reuters) - Brazilian judge Alexandre de Moraes' threats to shut down billionaire Elon Musk's social media platform X are just the latest chapter in a no-holds-barred crusade against perceived attacks on democracy and the political use of disinformation.

"We have a right to defend fundamental rights. Those who violate democracy, who violate fundamental human rights, whether in person or through social media, must be held accountable," Moraes said in a speech on Friday, without mentioning X by name or his decision.

Musk, who is also CEO of electric-vehicle maker Tesla and rocket company SpaceX, has denounced the move as censorship. 

Moraes, 55, is a lawyer who was tough on fighting crime when he was in charge of public security for the state of Sao Paulo. He shot to prominence in 2019, two years after his appointment to the Supreme Court, when he began to lead an investigation into "fake news" being generated during the government of former far-right President Jair Bolsonaro.

That investigation, which is still open, focused on what Moraes himself has described as a "cabinet of hate" that operated from inside Bolsonaro's presidential palace with the aim of attacking opponents and spreading lies and distortions.

"Brazilian institutions were failing to regulate social media properly," said André Cesar, a political analyst. "Moraes took on the role of grand regulator and he likes doing that." 

Among the falsehoods investigated by the judge was criticism by Bolsonaro and his political associates that Brazil's electronic voting system was open to manipulation.

Moraes was also in charge of investigating an alleged conspiracy to provoke a military coup to overturn Bolsonaro's defeat by leftist President Luiz Inacio Lula da Silva in 2022 and Bolsonaro's encouragement of his supporters to invade and vandalize government buildings a week after Lula took office.

Under his leadership, the country's electoral authority declared Bolsonaro ineligible for eight years, the first time this happened to a Brazilian president in the country's history.

While Moraes' defenders see him as a crusader in the defense of democracy, critics accuse him of using heavy-handed methods to raid the homes of politicians and businessmen, arrest them without trial and freeze their bank accounts.

Musk, who has compared Moraes to the "Harry Potter" villain Voldemort, has also described the judge as a "dictator," calling him "an outright criminal of the worst kind, masquerading as a judge." 

In his fight against disinformation, the judge ordered social media networks to take down postings by politicians and influencers that he deemed to be a threat to Brazil's institutions, usually on accounts of Bolsonaro backers who denied he had lost his 2022 reelection bid.

And that is where his run-ins with Musk's platform began.

After Musk challenged his decision to bar certain accounts and said he would reactivate them, Moraes in April opened an  inquiry into the billionaire for obstruction of justice.

Earlier this month, X announced it was closing its offices in Brazil due to what it called "censorship orders" from Moraes, though it kept its site open for Brazilian users.

The judge threatened on Wednesday to suspend the service if X did not appoint a legal representative as required by Brazilian law.

On Thursday, Moraes blocked the local bank accounts of Musk's Starlink internet firm, saying they could be used to cover hefty fines that the judge previously imposed on X.

Even as many Brazilians have sided with Moraes, insisting that Musk must comply with Brazilian law, some have asked if he has gone too far.

"He is a defender of democracy, but has overstepped his role as a judge at times dealing with social media," said analyst Cesar, of consulting firm Hold Assessoria Legislativa. "He thinks public opinion is on his side, but I don't know for how long."

Before sitting on the Supreme Court, Moraes was minister of justice after serving as secretary of public security in his home state of Sao Paulo.

Moraes, a Catholic, holds conservative views on the economy. For years, he was a member of the centrist Brazilian Social Democracy Party which has largely vanished from the political landscape.