"I am not Chavez," Maduro said, wearing a simple red shirt. "In terms of intelligence, charisma, historical force, or capacity to lead. ... But I am a Chavista and I live and die for him."
Opposition critics have called Maduro's ascension unconstitutional, noting the charter designates the National Assembly president as acting leader if a president-elect cannot be sworn in.
Capriles faced a stark choice in deciding whether to compete in the vote, which most analysts say he is sure to lose amid a frenzy of sympathy and mourning for the dead president.
Some say a second defeat for Capriles just six months after he lost last year's presidential vote to Chavez could derail his political career. But staying on the sidelines would have put in jeopardy his leadership of the opposition.
Analysts predict the next five weeks will see a spike in the nasty, heated rhetoric that began even before Chavez's death Tuesday after a nearly two-year fight with cancer.
Political consultant Oswaldo Ramirez, who is advising the Capriles campaign, said the candidate must strike a balance between criticizing the failures of Chavez's government and Maduro's role in it, without being seen as attacking the late president.
"He can't speak badly of Chavez, because this feeling on the street is still in full bloom," Ramirez said.
Public opinion was as divided as always Sunday in a country that became dramatically more polarized during Chavez's 14-year rule.
"It's not fair," said Jose Mendez, a 54-year-old businessman of the choice the opposition leader faces. "(Maduro) has an advantage, because of everything they have done since Chavez's death, all the sentiment they've created ... But the guy has nothing. He can't hold a candle to Chavez."
But Ramon Romero said the opposition was just making excuses, and had no chance of victory in any case.
"Now their odds are even worse," said the 64-year-old waiter and staunch Chavez supporter. "They don't care about anyone, and we (the voters) have been lifted out of darkness."