The candidate, who is running against President Hugo Chavez in the country's Oct. 7 election, used the 45-minute video appearance to field questions sent via Facebook and Twitter.
Answering one of the questions, Capriles criticized Chavez's decision this week to pull out of the Inter-American Court of Human Rights, calling it "irresponsible."
"He doesn't like them to put limits on him," Capriles said, referring to Chavez. "This affects those Venezuelans who are left without justice in their country."
Capriles said that if he's elected, he would restore Venezuela's membership in any such international bodies.
He spoke on Wednesday night before an audience in a studio at the Venezuelan television channel RCTV, which Chavez's government forced off the airwaves in 2007. The government had refused to renew the channel's license, accusing it of violating broadcast laws and backing a failed 2002 coup against Chavez.
Capriles said if he wins the vote, he would allow RCTV to go back on the air.
Both Capriles and Chavez are active on Twitter, but the live video talk on Facebook was a first for a Venezuelan politician.
Andreina Marquez, who leads online efforts for Capriles' campaign, said the online forum was a way to "open another channel of participation."
A poll last month by the Caracas-based polling firm Datanalisis found that Chavez had a 15-point lead over Capriles. The survey, which had a margin of error of 3 percentage points, also found that 23 percent of those polled didn't express an opinion as to which candidate they would support.