SAO PAULO - Human rights groups and researchers have raised concerns in Brazil that social media platforms are failing to effectively police disinformation ahead of a highly polarized presidential vote on Sunday.

Brazil's Superior Electoral Court (TSE) bolstered measures this month to tackle disinformation around the election, especially on video sharing platforms. Sunday's runoff vote pits far-right President Jair Bolsonaro against leftist former President Luiz Inacio Lula da Silva.

Groups that monitor the online spaces told Reuters that tech companies are still struggling to follow through on agreements they signed with the TSE in February, in which they promised to combat material that could undermine the democratic process.

The TSE did not respond to a request for comment.

"Social media platforms are failing Brazil's voters," said Deborah Brown, a senior researcher on digital rights at Human Rights Watch, who called the platforms and messaging apps "extremely important" spaces for electoral debate.

"That space has been riddled with electoral disinformation, such as baseless allegations of electoral fraud," she said.

Bolsonaro, who is narrowly trailing Lula in recent opinion polls, has repeatedly criticized the country's electronic voting machines for allegedly being vulnerable to fraud, without providing evidence, raising concerns that he might not concede defeat if he loses.

Jon Lloyd, senior advisor at UK-based Global Witness, said the platforms have "many correct policies" in place to combat disinformation but "the enforcement is really lacking."

"They're not actually following through on what their policies say and that's a big concern," he said.

Online disinformation has intensified since Brazil's first round of voting on Oct. 2, according to Philip Friedrich, a senior research analyst for technology and elections at Freedom House.

"Social media companies bear some responsibility for the country being on tenterhooks about this year's election," Friedrich said. "While major social media companies like Twitter and Meta released statements about their preparations before the first round of voting, we've seen few updates about how they are responding to shifts in disinformation."

An explosion of video-based platforms has also added to the difficulty of fighting disinformation compared to past elections, said Brazil researcher Rosana Pinheiro-Machado at Dublin's University College.

"The last election was the election of WhatsApp. This is the election of Tiktok and Instagram," she said.

TikTok said in a statement to Reuters "we take our responsibility to protect the integrity of our platform and elections with utmost seriousness."

A spokesperson for Meta Platforms Inc, parent company of Facebook, Instagram and WhatsApp, said: “We’ve prepared extensively for the 2022 election in Brazil," adding that around 30 million people in Brazil have clicked on the electoral label on Facebook and have been directed to the TSE's website.

Twitter and YouTube, which is owned by Google parent Alphabet, did not immediately respond to requests for comment.

(Reporting by Steven Grattan Editing by Frances Kerry)