Capriles, 39, easily won Sunday's primary elections to be the opposition's sole candidate but now faces a tough battle against the firebrand president.
"There's the face of the candidate of the bourgeois, of capitalism, of imperialism," said Chavez in a public act, referring to Capriles without mentioning his name.
"Bourgeoisie, you want to be like Chavez," he said, refering to a speech in which Capriles defended a government social program -- part of popular policies which have helped keep Chavez in power for some 13 years.
Capriles, a moderate who describes his politics as center-left, has argued that Venezuela can "replicate" Brazil's model of economic development that mixes free-market economics while prioritizing social progress.
Chavez, 57, a leftist ally of Cuba and harsh critic of the United States, remains popular among the working class, backed by resources including state media, food and fuel subsidies.
"I'm radically on the left, socialist, revolutionary, anti-imperialist," Chavez said Wednesday.
Capriles, the governor of Miranda state, defeated five other candidates Sunday in the first-ever primary held by the traditionally fractured opposition. Voters also selected opposition candidates for local and state-level elections.
More than three million people, or 17 percent of the electorate, took part, surpassing expectations.
Chavez on Wednesday attacked the opposition's refusal to present voter lists despite a court order, saying it was "very serious" and "sounds like fascism."
The Supreme Court intervened Tuesday after a complaint from a local mayoral candidate who wanted to challenge the result.
But the Democratic Unity (MUD) coalition said the request was "unconstitutional" and that records had been destroyed to meet a promise to ensure confidentiality.
There were also fears that those who voted could face government reprisals following an unsuccessful recall referendum against President Chavez in 2004.
Many people who backed that initiative complained they had suffered discrimination in public posts after their names were revealed.