They are boycotting the "Constituency Assembly" election, claiming President Nicolas Maduro will use its result to crush dissenters and tighten his grip on power.
Polling stations are to open at 7am local time (12pm BST) and are expected to close 11 hours later.
If successful, President Maduro will essentially govern a one-party state, controlling the powerful 545-member body charged with re-writing the Venezuelan constitution.
The vote will mark the peak of four months of protests, that have seen 110 people killed in clashes with riot police.
Opposition to President Maduro has steadily grown beyond its roots in the middle classes to include significant support from the slums in Caracas.
"This is illegal," Juan Vieda, a man staffing a barricade in the capital, told Sky News.
"They want to force us. They want to force this constitution on us and we don't want it. This will be bad and this will be bad for them."
President Maduro is going all out to boost turnout and increase the election's legitimacy.
State employees, who depend on their free housing and food subsidies, have been bombarded with texts and calls telling them to vote or risk losing their lifelines.
All radio stations have also been ordered to play pro-election adverts at specific times set by the government.
Managers of state-owned businesses have even been told to guarantee their staff turn out or face losing their jobs.
For workers in the private sector, the self-employed and the huge numbers of unemployed, the threats are less potent. 70% are against the election.
Opponents hope a tiny turnout will expose the illegitimacy of the process.
But the government controls the election returns and there will be no independent oversight.
President Maduro insists the new assembly is the only way to haul Venezuela out of its economic and political crisis, but has not explained how a new constitution would do so.
"We have a card to play: a card that will win this game. And that card is the National Constituent Assembly," he said on Friday.