Witnesses said some people in the caravan had gotten out to try to convince the Chavez supporters to let them through when gunfire rang out. Two Capriles supporters died in the violence on Saturday in the western state of Barinas and a third was seriously wounded.
There have been other spasms of violence in the heated campaign ahead of Venezuela's Oct. 7 election, but this was the first fatal incident, and it sharply ratcheted up tensions.
Capriles condemned the violence on Sunday, saying at a campaign rally: "The time of hatred is going to be buried in Venezuela." Chavez also called for calm, saying: "It's not with violence that we're going to face each other. It's vote against vote."
Justice Minister Tareck El Aissami said on Sunday in a message on Twitter that one suspect was arrested in the killings, though he didn't identify the suspect or say how he was captured.
Opposition politician Pedro Castillo, who was in the caravan and witnessed the violence, said there was no physical confrontation before the shooting erupted. He said the caravan was stopped in a long line of cars when some of the Capriles supporters got out, and that more than two dozen Chavez supporters had laid tires in the road to block
A video posted online showed the two groups arguing.
One opposition supporter, 32-year-old Jason Valero, was among those standing in the street and tried to walk through the barricade, Castillo said. Then, he was shot in the chest, Castillo said.
"There wasn't even a shove," Castillo said. "The matter went from something verbal to taking out a gun and despicably killing a person, and then to starting with a barrage of gunshots."
The video posted on YouTube showed people scattering as the shots rang out.
An older opposition supporter, Omar Fernandez, had been blowing on a horn of the sort used in political rallies, Castillo said. Fernandez was wounded in the neck and died at a hospital, he said.
Fernandez had been an agricultural secretary for the opposition party Un Nuevo Tiempo in the town of Barinitas, said opposition politician Rafael Riera, who also witnessed the shooting.
Valero was a father of four who worked in a family cargo truck business, Castillo said.
A third man, local campaign photographer Hector Rojas, was seriously wounded in the shooting, while Castillo said a teenage girl suffered a less serious wound and was released from a hospital.
Riera said the people in the caravan had simply wanted to make a swing through the area and had tried to tell the Chavez supporters "we didn't want any sort of problems."
Riera said that people had seen a truck nearby and "from there some individuals got out and fired some shots."
Both opposition politicians said that after the shooting one suspect was stopped by people before he could get away, and was then detained by authorities. They also said a truck was burned after the shooting.
"What we want is what their relatives are demanding... that justice be done and that this act not go unpunished," said Castillo, who attended wakes for the victims.
In another previous outburst of campaign-related violence, at least 14 people were injured on Sept. 12 when stone throwing broke out after Chavez backers blocked a road trying to prevent Capriles from reaching an event in Puerto Cabello.
In March, shots erupted while Capriles was visiting a Caracas district that traditionally has been pro-Chavez, and one opposition supporter was wounded.