(Adds Bolsonaro retort, WhatsApp comment)
By Lisandra Paraguassu and Eduardo Simões
BRASILIA/SAO PAULO, Oct 18 (Reuters) - Leftist presidential candidate Fernando Haddad accused right-wing front-runner Jair Bolsonaro on Thursday of soliciting mass messaging over social media via WhatsApp, funded by businesses, in an alleged violation of Brazil's campaign finance law.
Haddad denounced the practice after newspaper Folha de S.Paulo reported that well-heeled supporters paid for messaging by third-party agencies, each paying up to 12 million reais ($3.26 million) to spread tens of thousands of attack ads.
Bolsonaro called it "voluntary support" in a Twitter post retort that accused Haddad's Workers Party (PT) of stealing public money to "buy minds" for political gain while in office.
Haddad said his party has witnesses saying Bolsonaro asked business leaders at a dinner in Sao Paulo for funds to pay for the bulk messaging, which he described as undeclared campaign contributions.
In a radio interview, Haddad said the Bolsonaro campaign had "created a veritable criminal organization with businessmen who are using undeclared money to pay for false messages on WhatsApp."
PT lawyers asked prosecutors and Brazil federal police to investigate. Illegal under-the-table funding by companies could, if proven, lead to the disqualification of the Bolsonaro ticket, said attorney Guilherme Salles Gonçalves, an electoral law expert.
PT allies said they would file a complaint with electoral courts. Haddad called on international observers and WhatsApp itself to look into the case.
A representative for WhatsApp, a division of Facebook Inc , said the report was being taken seriously.
"WhatsApp has proactively banned hundreds of thousands of accounts during the Brazilian election period," a spokesperson said. WhatsApp's spam detection technology spots accounts that engage in automated behavior and spread misinformation, she said.
Campaign propaganda has flooded social media in Brazil ahead of an Oct. 28 run-off vote in the presidential election, which Bolsonaro is expected to win.
Haddad replaced jailed party founder Luiz Inacio Lula da Silva on the PT ticket after the former president was banned from running due to a corruption conviction.
Riding high on an anti-graft and law-and-order platform, Bolsonaro won 46 percent of valid votes in the first-round election on Oct. 7, barely missing an outright victory.
With a growing lead in opinion polls, Bolsonaro has said the presidency is "within reach," although he has been unable to campaign due to a near-fatal stabbing during a rally last month.
His doctors had instructed him not to take part in debates against Haddad but said after a check-up on Thursday that the decision was up to him and his campaign. They said he is recovering well but still needs "nutritional support and physiotherapy." (Additional reporting by Gabriel Stargardter in Rio de Janeiro, Ricardo Brito and Anthony Boadle in Brasilia Editing by Dan Grebler and Bill Trott)