Henrique Capriles, 40, accused Maduro in remarks broadcast on state television Sunday night of lying to the country about the timing of the death of the president -- who hadn't appeared in public since December.
"Nicolas, already in [the] campaign, has been lying to Venezuelans," Capriles said.
"Who knows when President Chavez died?"
The government said Chavez died of cancer complications Tuesday, but never said what type of cancer he had. Some diplomats have suggested Chavez died much earlier.
Capriles said Maduro was working with others in government to "coldly" calculate every detail associated with Chavez's death and the electoral process moving forward down to the "millimeter."
Maduro, 58, responded by going on national television to declare that Chavez, who Maduro said last week would be embalmed and put on display at Caracas' Museum of the Revolution, would now be displayed at the National Pantheon, given the popular outcry for having Chavez's final resting place be with Venezuelan heroes including Simon Bolivar, champion of independence from Spain.
Maduro said the National Assembly agreed to bypass a constitutional provision that says no one can be placed in the pantheon for at least 25 years after death. Chavez will now be placed there as soon as possible, he said.
"It is a cry of the people, everyone, everywhere," Maduro said. "And we will do it. Chavez to the Pantheon, next to Simon."
For his part, Chavez long said he wanted to be buried in the grasslands of his native Barinas state.
Maduro called Capriles a "fascist" who defiled the name of Chavez.
"We reject an infamy that you plan to hurl and the words you've said about the crystalline, pure image of Commander Chavez," Maduro said. "Enough of the offenses, sir!"
He also said Capriles' decision to run against him would prove to be "the biggest mistake" in his career, adding, "You do not know what you're getting into" and calling him a "losing, miserable candidate."
Capriles -- a law graduate and former national legislator who is now governor of Miranda, Venezuela's second most-populous state -- said Sunday night he would formalize his candidacy Monday and start touring the country Tuesday.
Venezuela's election commission during the weekend set the vote for April 14.
"I am going to fight," Capriles said at a news conference. "Nicolas, I am not going to give you a free pass. You will have to beat me with votes."
Capriles lost to Chavez in Oct. 7 elections by 11 percentage points, capturing 44 percent of ballots.
Though unsuccessful, his win was the best showing by a candidate ever against Chavez, who led Venezuela for 14 years.
Election authorities designated only 10 days for campaigning, a short time frame The Wall Street Journal said likely favors Maduro, who is backed by the electoral machine of the ruling United Socialist Party of Venezuela, or PSUV.
Chavez, in his final public appearance in December, backed Maduro as Venezuela's next leader if a new vote was required.