STORY: Volunteers started allocating voting machines early in the morning in Sao Paulo. Regional electoral court official Cintia Nakasa said each box may be used by up to 400 voters.
Brazil adopted electronic voting machines in 1996 to end widespread fraud involving paper ballots.
After winning political office in a dozen elections using the current voting system, Bolsonaro ramped up his criticism last year as opinion surveys showed him on track to lose re-election.
He has repeatedly asserted that the machines are open to tampering, but he has produced no evidence of fraud.
International voting experts and electoral officials say the technology is secure, with no cases of fraud detected.
Still, some election experts say the lack of a paper record for each vote does limit opportunities to conduct audits if the election is contested.
Voters punch their choices into electronic voting machines at 477,000 voting stations across Brazil. A paper receipt of each machine's total is signed by officials and displayed publicly in each polling place.