Former President Otto Perez Molina resigned this week and was swiftly arrested over a customs fraud scheme.
Many people in the Latin American country have called for the polls to be postponed in the wake of the allegations.
Leading contenders include centre-right businessman Manuel Baldizon and comic actor Jimmy Morales.
If no candidate wins 50% of the vote there will be a second round of voting on October 25.
There have been calls for Guatemalans to wear black clothes of mourning as they cast their vote, amid skepticism that the poll can achieve political change.
Prosecutors accuse Otto Perez Molina of masterminding a scheme in which businessmen paid bribes to evade customs charges. The former president denies involvement.
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