Venezuelan opposition leader Maria Corina Machado and her coalition's candidate Edmundo Gonzalez have commanded enthusiastic crowds during their campaign to unseat President Nicolas Maduro and end 25 years of ruling party dominance.

But uncertainty remains about how Sunday's election will play out, with Maduro – whose 2018 reelection is considered fraudulent by the United States, among others - sounding a confident note and opposition figures and analysts warning of possible underhandedness.

Gonzalez has attracted significant support, even from former supporters of the ruling party, but the opposition and some observers have questioned whether the vote will be fair, saying decisions by electoral authorities and the arrests of some opposition campaign staff are meant to create obstacles.

Gonzalez and Machado have urged voters to cast ballots early and hold "vigils" at polling stations until they close. They have said they expect the military to uphold the results of the vote.

Venezuela's military has long supported Maduro and his predecessor, the late Hugo Chavez. Defense Minister General Vladimir Padrino has said the armed forces will respect the outcome of Sunday's election.

Maduro, who has been in power since 2013 and is seeking his third six-year term, has said the country has the world's most transparent electoral system and has warned of a "bloodbath" if he were to lose.

Maduro's government has presided over an economic collapse, the migration of about a third of the population and sharply-deteriorated diplomatic relations, crowned by sanctions imposed by the United States, European Union and others which have crippled an already-struggling oil industry.