Facebook Inc’s WhatsApp messaging service became the frontline in Brazil’s bitter presidential campaign on Friday, as front-runner Jair Bolsonaro angrily denied accusations he had encouraged widespread misinformation campaigns on the platform.

WhatsApp has now banned hundreds of thousands of accounts in Brazil as the Facebook Inc. messaging service struggles to contain spam, misinformation and other political shenanigans ahead of a runoff election in Latin America’s largest country.

On Thursday, newspaper Folha de S.Paulo had reported that supporters of the far-right candidate had funded mass messaging attacks against leftist rival Fernando Haddad.

“We have cutting-edge technology to detect spam that identifies accounts with abnormal behavior so that they can’t be used to spread spam or misinformation,” a WhatsApp spokesperson wrote in an email. “We are also taking immediate legal measures to prevent companies from sending mass messages via WhatsApp and have already banned accounts associated with those companies.”

Facebook set up a “war room” to spot and eradicate misinformation, hate speech and other damaging content during Brazil’s election this month. It’s a test for the social network ahead of midterm elections in the U.S. later this year.

While the company said it was able to thwart false information on its main social network, it’s had more trouble controlling misbehavior on WhatsApp, which is encrypted and virtually impossible to monitor.

There are around 120 million users of the messaging service in Brazil, whose population is 210 million.

Brazil’s runoff election is on October 28, when far-right politician Jair Bolsonaro faces leftist Workers’ Party candidate Fernando Haddad.

On Thursday, the latter accused his competitor of receiving illegal and undeclared corporate donations after local newspaper Folha de S.Paulo reported companies are paying for a mass social media campaign against Mr Haddad. Mr Bolsonaro wrote on Twitter that his opponent’s party “isn’t being damaged by fake news, but rather by the TRUTH”.

WhatsApp said on Friday that it’s taking the accusation seriously. “We are committed to reinforcing WhatsApp policies equally and in a fair way to protect the users’ experience,” it added.

The messaging service confirmed that it blocked the account of Mr Bolsonaro’s son Flavio for spam a couple of days ago.

The company said it was unrelated to the accusations on Thursday.

It also noted that it banned an account related to former Brazil president Dilma Rousseff, a member of the Workers’ Party.

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Nigeria own go be 700k accounts wey dem go shutdown....come 2019..many don start whatsapp campaigns already.

The way the 2019 elections are going. For sure millions of accounts will be banned by Whatsapp. All the misinformation and spam is already happening on whasapp groups. The administrators also have to be very careful about allowing those postings to be made.