Capriles is trailing in the polls in a race that so far has been dominated by speculation about Chavez's health. The president has scaled back his public appearances after finishing his latest cancer treatments in Cuba, and it remains to be seen how active Chavez will be during the campaign.
"I will do my duty to Venezuela," Capriles told hundreds of supporters.
He spoke at a stadium in Caracas as he formally handed over his office vowing to become Venezuela's next president. He entrusted the governor's post to an aide, Adriana D'Elia.
The 39-year-old Capriles was chosen in a February primary to be the opposition's single challenger to take on Chavez. The leftist president, who was first elected in 1998, is seeking another six-year term.
Capriles said if he wins the Oct. 7 vote, one of his priorities will be to seek unity and leave behind the polarized political climate that has become the norm during Chavez's presidency.
Capriles didn't mention Chavez by name during his speech, but said in reference to his rival's style: "I don't believe in insults." Chavez has often called his opponent "el majunche," a term used by Venezuelans meaning mediocre or poor quality.
So far, Capriles has been trailing in the polls, though survey results have varied widely.
During Chavez's nearly yearlong struggle with cancer, his illness hasn't had a significant effect on his support and he has remained above Capriles.
Political analyst Ricardo Rios said that while Capriles is behind in the polls, the opposition leader could see improvement in the coming months. He noted that some surveys indicate more than 20 percent of voters haven't yet committed to either candidate.
Rios said Capriles' energetic campaigning also is an asset up against Chavez, who recently hasn't been out rallying supporters as he often used to do.
In the past year, Chavez has undergone two surgeries that removed tumors from his pelvic region, most recently in February. He returned home from Cuba on May 11 after what he said was his latest round of radiation therapy, and since then has limited his public appearances while saying little about his illness.
He appeared seated in some televised appearances last month, leading to new speculation about diminished strength and mobility. Since Saturday, he has twice appeared on television standing and walking at the presidential palace.
Chavez's campaign manager, Jorge Rodriguez, said Wednesday that Chavez will appear in person Monday to register his candidacy at the office of the National Electoral Council.
Capriles' supporters plan a march Sunday as the opposition leader signs up as a candidate.
Jusmeiby Sojo, one of Capriles' supporters at the rally, said Chavez hasn't delivered in his more than 13 years in office.
"We've seen pure demagoguery, pure talking and talking, and we want actions," she said.