Maduro's opponents are vowing to stage the "mother of all protests" calling for his ouster, after two weeks of violent demos that have left five people dead and dozens wounded.
Sowing fears of more violence, Maduro is in turn urging his supporters, the military and civilian militias to defend the leftist "revolution" launched by his late predecessor Hugo Chavez in 1999.
It is set to be the biggest day of protests since Maduro's allies moved to strip the power of the opposition- majority legislature -- the only lever of government they do not control -- and banned opposition leader Henrique Capriles from politics.
The protests have been relatively small so far, with turnout in the thousands.
The opposition is now hoping to flood the streets with a sea of protesters, whom they are urging to remain peaceful.
They plan to march from 26 rally points toward central Caracas, a pro-Maduro bastion and the seat of government. The authorities say they will not allow them into the area, where the rival rally will be held.
Wednesday is a national holiday that marks the start of Venezuela's independence struggle in 1810.
Maduro's camp vowed not to be outdone by the opposition.
"The whole of Caracas will be held by the revolutionary forces," said lawmaker Diosdado Cabello, one of the president's most powerful allies.
Pressure on Maduro has risen in an economic crisis aggravated by a fall in prices for Venezuela's crucial oil exports, which has triggered severe shortages of food and medicine.
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