As flames engulfed Notre-Dame cathedral in Paris on Monday night, far-right activists spread rumours and conspiracy theories about the cause of the fire.

Many sought to link the fire with Muslims, despite officials declaring arson highly improbable.

Twitter has faced widespread criticism over the spread of white nationalist propaganda on its platform.

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As a huge fire took hold of Paris' Notre-Dame Cathedral on Monday evening, alt-right figures were in no time spreading rumours and disinformation on social media linking the blaze to Muslims and hinting at a sinister cover up.

French officials on Monday night were quick to say that arson was unlikely to be the cause of the blaze as it engulfed the roof of the 12th century cathedral, but far-right activists and propagandists were already all over social media channels pushing conspiracies.

Conspiracy theorist Jack Posobiec compared the blaze to 9/11, despite no link having made to terrorism by French officials, while alt-right activist Faith Goldy falsely claimed that three days previously "a Muslim jihadis [sic] in Paris was arrested for planning a terrorist attack at Notre-Dame Cathedral.

"Today, Notre Dame is in flames. Probably just a coincidence," she continued.

Goldy linked to a report on a woman being jailed three weeks previously for planning an attack at the church in 2016, with hundreds of other accounts spreading the tweet and linking to reports on the plot, without noting that reports were years old.

Paul Joseph Watson, of the InfoWars website, tweeted that one worker was "claiming that the blaze was deliberately set," linking to a report he wrote he said was based on a tweet by Time columnist and Fox News contributor Christopher J. Hale, which Hale later deleted, describing it as an "unsubstantiated rumor."

Watson provided no other evidence for the claim.

—Ryan Broderick (@broderick) 15 April 2019

"IMO, it's either massive carelessness - or arson," tweeted alt-right activist Stefan Molyneux, who groundlessly alluded to a cover up, subsequently remarking: "Notre-Dame is going to go down the memory hole. Unless something leaks, nothing will be explained."

As the rumours and speculation took hold and spread, media organisations and populist politicians helped to broadcast them to an even wider audience.

Russian state funded broadcaster RT — which has frequently provided a platform for the far right — published a story headlined "Notre-Dame fire hits after several cases of vandalism against French Catholic churches."

Separately, right-wing populist website Breitbart published a tweet by French far-right activist Damien Rieu, in which he claimed that Muslims had used laughter emojis in comments on a Facebook report on the blaze. The tweet was also promoted by Watson.

Alice Weidel, parliamentary leader of the far-right German AfD party, citing no evidence, linked the blaze to "discrimination against Christians," tweeting "during Holy Week Notre-Dame burns. March: second largest church Saint-Sulpice burns. February: 47 attacks in France."

"The Observatory on Intolerance and Discrimination against Christians in Europe speaks of a significant increase [in attacks against Christians]," she added, linking to a report on recent desecration of churches in France.

Twitter is under pressure from lawmakers in Europe and the US to halt the spread of disinformation and hate speech on its platform, and critics last night called on the site to act as disinformation spread.

CNN executive Matt Dornic said that Twitter had initially refused to take action against an account spreading a fake CNN story linking the fire to terrorism, claiming it was a parody.

—Matt Dornic (@mdornic) 15 April 2019

"The team is reviewing reports and if they are in violation suspending them per the Twitter Rules," a Twitter spokesperson told Business Insider.

"Our focus continues to be detecting and removing coordinated attempts to manipulate the conversation at speed and scale."