Opinion polls suggest that conservative opposition candidates are in the lead - a change of power after almost 20 years of left-wing rule appears possible.
The political scene has been characterised for months by the power struggle between former president Evo Morales and his successor, Luis Arce.
Outgoing president Arce is not seeking re-election, while Morales has been barred from running by court order.
Bolivia, one of South America's poorest countries, is grappling with a deep economic crisis marked by foreign currency shortages, fuel scarcity, inflation, and growing public discontent.
This discontent could boost the opposition, with conservative ex-president Jorge Quiroga and businessman Samuel Doria Medina seen as leading contenders.
On the left, Senate President Andrónico Rodríguez and former interior minister Eduardo del Castillo are considered the most promising candidates.
Observers expect that no candidate will achieve the necessary majority to win in the first round of voting on Sunday. A run-off election is scheduled for October 19.
The winner will take over the presidency for a five-year term in November.
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