Tired and hoarse after more than a month protesting in choking tear gas, the leader of the center-right Justice First party, 44, vowed to keep marching and boycott Maduro's reform plans.
"Venezuela is heading towards an outcome. This is unsustainable," Capriles said in Wednesday's interview in Caracas.
"If there is a constitution that is the result of a minority imposing rules, the government will fall."
Constitutional row
Capriles said Maduro's latest maneuver to resist opposition pressure, by moving to overhaul the constitution, was a bid to avoid elections.
"Since the government cannot win elections, it wants to dismantle the system for holding them," he said.
Maduro was elected in 2013. He says an assembly of popular representatives will be elected to draft the new text. Capriles says it will be filled with Maduro's supporters.
"They say the vote will be universal... but only in the universe of those who they decide can take part," he said.
"I will not take part in what he is proposing under any circumstances... The situation in the country will become very complicated, with two constitutions, the one that is in force and a fraudulent one."
Street violence
Clashes between protesters and police have left 35 people dead in just over a month, according to a new toll given by prosecutors on Thursday -- but Capriles insisted the rallies must continue.
"I don't know how long the protests are going to last. What I can say is that I see admirable resolve in the people," he said.
"If we were being violent, if we were not being democratic, we would already have toppled the government."
The government and opposition accuse each other of paying armed groups to stir up violence in the protests.
"Maduro is playing his last cards. He is sending the most radical people" to deal with protests, Capriles said.
"We are not even sure whether those who are repressing the protesters are even National Guard (military police) or national police."
Military allegiance
Maduro, 54, has come under international pressure, with the United States and other world powers expressing concerns for democracy in Venezuela.
"This government is isolating itself from the democratic world, without financing, without resources, with the highest inflation and shortages in the world."
The tide could turn against Maduro if the military high command stops backing him.
"What happens if the National Guard says they are not going to continue the repression? Will Maduro's position be sustainable?" Capriles said.
He claims the military rank and file do not all support Maduro and has tried to rally the armed forces to support the constitution.
"We are calling for that internal debate and division to yield not a coup or an uprising but rather for the democratic armed forces to impose themselves," he said.
"That way Maduro will not be able to continue using them for repression."
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