A fact-checking group said that Fernando Haddad has been targeted by a wave of false allegations,some accusing him of planning to shut down churches and distribute textbooks teaching children to be gay.
Haddad blamed his adversary,far-right candidate Jair Bolsonaro,for planting or failing to condemn the falsehoods.
"The lies come from him[Bolsonaro],so he will not accept any ethical commitment. He will continue to slander,insult,"Haddad said.
He said his campaign had managed to have 33 videos removed from social media sites."One had 3 million views when it was removed. We try to stop it,but people watch them."
Haddad has asked Bolsonaro to jointly sign a commitment against spreading fake news before the run-off,but Bolsonaro rejected the idea,calling Haddad"a scoundrel"in a Twitter post.
Haddad said on Tuesday the refusal was"a proof of dishonesty".
Bolsonaro,a former army captain,won Sunday's first round of voting with 46 per cent.