Haddad also promised to bring conciliation if he beats front-runner Jair Bolsonaro, who stayed in his home in Rio de Janeiro speaking to his voters via social media.
Haddad, who polls indicate is the underdog in the runoff, spoke during a rally in Heliopolis, Sao Paulo's biggest favela with more than 100,000 residents. Violent areas of big cities gave Bolsonaro less votes in the Oct. 7 first-round than safer and wealthier areas.
"Arming the population, like my adversary suggests, will only increase violence. Can you imagine children and women bearing guns too? My adversary's ideas have already been tested in other countries and the number of homicides only picked up," Haddad said.
Later the candidate hand-picked by jailed former President Luiz Inacio Lula da Silva said this Brazilian election would be different from any other if Bolsonaro loses at the last minute.
"That would be the victory of a project, not of a person or a party. It would be a vote for democracy and freedom," Haddad said during a discussion in social media.
Two polls published later on Saturday showed Bolsonaro's advantage falling once more, but still comfortable enough to win.
A Datafolha poll showed Bolsonaro with 55 percent and Haddad with 45. The pollster interviewed 18,371 voters on Friday and Saturday. The margin of error is of two percentage points.
Haddad suffered a blow when defeated presidential candidate Ciro Gomes, a leftist who finished the first-round in third place with 12 percent of the vote, failed to strongly support the Workers' Party candidate.
"Everybody wanted me, with my style, to pick a side and take part in the campaign," Gomes said in a video. "But I don't want to do this now for a reason that is very practical and I don't want to say it now. If I can't help, I don't want to get in the way."
Without a clear endorsement from Gomes, Haddad made a surprising nod to right-leaning defeated presidential candidate Geraldo Alckmin, saying he gave the inspiration to one of his proposals to cut cooking gas prices.
The left-leaning candidate also received two surprising endorsements from a former chief justice and an ex-top prosecutor who took harsh measures against Haddad's Workers' Party. Joaquim Barbosa and Rodrigo Janot said Bolsonaro is a threat to Brazil's democracy.
Although Bolsonaro kept quiet at his home in Rio de Janeiro most of the day, his legal team was active. The far-right candidate filed a suit in Brazil's top electoral court requesting an investigation into Haddad and newspaper Folha de S.Paulo.
Bolsonaro claims the daily falsely reported that businessmen illegally sponsored WhatsApp messages against Haddad to help Bolsonaro.
Folha's attorney called Bolsonaro's request "a ridiculous fantasy."