RIO DE JANEIRO — Shortly after President Donald J. Trump was banned from Twitter early this year, Brazil’s like-minded leader made a plea to his millions of followers on the site.
“Sign up for my official channel on Telegram,” President Jair Bolsonaro requested.
Since then, Telegram, an encrypted messaging and social media platform run by an elusive Russian exile, has racked up tens of millions of new users in Brazil.
Its growing popularity in Brazil and elsewhere is being fueled by conservative politicians and commentators for whom it has become the most permissive disseminator of problematic content — including disinformation — in a social media ecosystem facing mounting pressure to combat fake news and polarization.
While WhatsApp remains by far the dominant messaging platform in Brazil, Telegram is making inroads fast. By August, it had been installed in 53 percent of all smartphones in Brazil, up from 15 percent two years earlier, according to a report.
Founded in 2013, Telegram has become a tool coveted by activists, dissidents and politicians — many in repressive nations like Iran and Cuba — to communicate privately.
But Brazilian government officials and experts worry the app could become a powerful vector for lies and vitriol before next year’s presidential elections — a tense political moment in the country.
Mr. Bolsonaro, his re-election prospects endangered by his diminishing popularity, has followed the Trump playbook and begun sowing doubts about the integrity of Brazil’s voting system, raising the possibility of a disputed outcome. His unfounded claim that electronic voting machines will be rigged has unnerved the opposition and the country’s top judges, who say the abundance of disinformation in Brazilian politics is doing lasting damage to its democracy.
“We know that systemic disinformation is produced by structures that are very well organized and financed,” said Aline Osório, a secretary general at Brazil’s electoral court who heads its program against misinformation.
Ms. Osório said the court had established constructive working relationships with executives from other social media companies that have become vehicles for misinformation campaigns. But its efforts to reach Telegram, which is based in Dubai, have been unsuccessful.
“Telegram has no representatives in Brazil, and this has made it difficult to establish a partnership in the same way we’ve done with other platforms,” she said.
Telegram did not respond to a request for an interview. Press queries are submitted through a bot on the platform.
Experts say political content and conversations have migrated substantially to Telegram in recent years in Brazil and other countries, largely because of the app’s capacity to mass-reproduce content.
Group chats can include up to 200,000 users, exponentially more than WhatsApp’s limit of 256. WhatsApp curbed users’ ability to forward messages after coming under criticism in Brazil and elsewhere for the role it played in misinformation campaigns during recent elections.
In addition to group chats, Telegram hosts channels, a one-way mass-communication tool used by corporations, artists and politicians to distribute messages, videos and audio files. Mr. Bolsonaro’s channel surpassed one million followers in recent weeks, putting him among the world’s most followed politicians on the platform.
While rival apps have adopted stricter and more clearly defined policies on abuse and disinformation, Telegram’s guidelines are vague, and the service takes a hands-off approach to content in individual and group chats.
That makes it a safe space for incendiary figures, including politicians, who have been banned from other platforms. In Brazil, the Twitter and Instagram accounts of a lawmaker, Daniel Silveira, and a conservative journalist, Allan dos Santos, were suspended as part of a Supreme Court investigation into disinformation campaigns that included threats against justices.
But Telegram remains a portal to their followers. That has enabled Mr. dos Santos to raise funds for his legal defense and call the justice who got him banned from other sites a “psychopath.”
“The network is clearly benefiting from the removal of users from other platforms,” Fabrício Benevenuto, a computer science professor at the Federal University of Minas Gerais, said of Telegram. “Politicians have noticed it makes no effort to remove accounts, so it is becoming an appealing network for more radical groups.”
Farzaneh Badiei, an internet governance expert who published a paper on Telegram at Yale Law School this year, said that Telegram’s founder, Pavel Durov, had been unwilling to meaningfully grapple with the problem of disinformation that goes viral.
“Their approach is very disorganized and very opaque,” she said. “We don’t see a systemic approach to solving these problems.”
Mr. Durov left Russia in 2014 after battling government efforts to censor content on the social networking site he founded, VKontakte. He has said he designed Telegram as an ultra private means of communicating based on the persecution he says he endured in his native country.
Twitter, Facebook, WhatsApp and YouTube played critical roles in Mr. Bolsonaro’s stunning victory in 2018, and the far-right leader has continued to rely heavily on social media to energize his base, attack opponents and make false claims that go largely unchallenged.
But in recent months, the platforms that enabled Mr. Bolsonaro’s rise have reined him in over his false or misleading claims about measures to contain the coronavirus. Social media companies put him on notice by taking down a handful of videos and tweets that they deemed dangerous.
Mr. Bolsonaro and his followers have railed against those removals as forms of censorship. In September, he argued that disinformation was now a permanent feature of politics and dismissed it as a trivial issue.
“Fake news is part of our life,” he said. “Who has never told a little lie to their girlfriend?”
Telegram has drawn critical scrutiny in Brazil for more than its disruptive role in politics. Investigations by news organizations found that it was hosting illegal arms networks and enabling the distribution of child pornography.
Brazilian lawmakers are debating legislation that would require platforms like Telegram to have legal representation in Brazil or risk being banned. However, users have easily circumvented such bans in countries like Iran and Russia by using software that lets them disguise their location.
Diogo Rais, a professor at Mackenzie University in São Paulo and a co-founder of the Digital Freedom Institute, called blocking apps a “drastic measure” that would be ineffective.
“We need to deal with digital challenges realizing that our laws are from 2009 and limited to our physical territory,” he said. “The digital world has no such limit. This is a global challenge.”