Chavez appeared tired, bloated and pale when he walked into the National Electoral Council to register his candidacy Monday. But later he exuded energy while singing along with a band playing a folk tune, and he seemed in his element as he delivered a fiery speech that lasted nearly three hours.
"We're just warming up our engines," Chavez said.
Then he took a jab at his rival, saying opposition candidate Henrique Capriles would "run out of gasoline."
The 57-year-old president has limited his recent public appearances after undergoing cancer treatment in Cuba, and he arrived at the elections office riding atop a truck.
Capriles, in contrast, flaunted his youthful energy Sunday by leading a huge crowd of supporters to the same elections office, working up a sweat as he walked and jogged six miles (10 kilometers) across the city.
Chavez said that despite his yearlong battle with cancer, he's ready to campaign and win by a "knockout" in the Oct. 7 election.
"We came from miracle to miracle, and I'm sure that with God's help we'll continue living and we'll continue triumphing," he said.
Chavez returned from Cuba on May 11 after what he said was a difficult round of radiation therapy. He has not disclosed details about his illness, including the type of cancer or the precise location of the two tumors that were surgically removed from his pelvic region during the past year.
"It was a difficult year," Chavez told the cheering crowd, which filled a plaza outside the elections office.
Some political analysts say Chavez has adroitly handled the issue of his illness ahead of the presidential campaign, which formally begins in July.
Carlos Blanco, a professor of Latin American affairs at Boston University, said Chavez has successfully manipulated public opinion to avoid being perceived as a moribund leader unfit to govern for another six-year term.
"Chavez has managed his illness with skill. He's gone from being the 'sick president' to the 'martyr president,' which allows him to maintain significant support," said Blanco, who served as Venezuela's minister for state reform from 1989 to 1992.
Polls say Chavez is leading Capriles. But Capriles has been actively campaigning and traveling across the country for several months seeking to drum up support.
Chavez said Saturday that he has undergone tests following his latest cancer treatment and everything came out well. Whether his illness would limit his ability to campaign has been a subject of speculation in Venezuela in recent weeks.
Luis Vicente Leon, who heads the Venezuelan polling firm Datanalisis, said the socialist leader has managed his illness in a secretive and clever manner that has allowed him to benefit politically, preventing many Venezuelans from believing "it's a deadly illness that will impede Chavez's future."
As the vote nears, though, Chavez will likely have to adjust his strategy and show he is well enough to campaign in order to avoid negative ideas about his health, Leon said.
For Capriles, one of his strong points against Chavez has been his relative energy. Supporters of the 39-year-old opposition leader have begun buying T-shirts with a new name for him, "The Skinny Guy President," in contrast to Chavez's burly build.
But Chavez's backers at Monday's rally played down the opposition leader's appeal.
"Capriles can jog all he wants. That's not going to convince people that he's capable of governing the country," said Rosa Morales, a 58-year-old housewife who praised Chavez's government for recently giving her family an apartment in a newly built housing project, allowing them to move out of a dilapidated shelter clinging to a hillside slum.
"Chavez, on the other hand, has been an excellent head of state - even being ill, and that's not going to change," she said.