Opposition officials said the two were shot in a confrontation with supporters of President Hugo Chávez, who is seeking re-election. The shooting occurred in Barinas, the state where Mr. Chávez was born and where he maintains strong support.

"Our people are tired of violence, of division, of confrontation," said the opposition candidate, Henrique Capriles Radonski, at a huge march that filled the streets of Caracas on Sunday. "There was no reason that these three young people had to fall, a result of the intolerance of a few."

He vowed to "defeat violence" in the election on Oct. 7.

Mr. Capriles referred to three deaths in his remarks, but the authorities have only reported that two people were killed. The reason for the discrepancy was not immediately clear.

Mr. Chávez, who has been in office for nearly 14 years, is seeking a new 6-year term. Mr. Capriles has mounted the strongest challenge to Mr. Chávez in years, tapping into widespread frustration with government inefficiency and corruption.

The country's justice minister, Tareck El Aissami, said in a post on his Twitter account Sunday morning that a suspect had been arrested in connection with the shooting, but he did not provide further information. In comments on state television Saturday, he called the shooting an "isolated event."

According to news reports, a group of Capriles supporters was about to begin a motorcade on Saturday afternoon when a group of Chávez supporters tried to block them. Some reports said the pro-Chávez group included local and state employees and officials. When one or more of the Capriles supporters approached them, someone on the Chávez side opened fire, according to the reports, which were based on interviews with Capriles campaign officials.

Julio César Reyes, a supporter of Mr. Capriles who is running for governor of Barinas, called the episode an ambush.

Mr. Capriles was campaigning in another state at the time of the shooting. He has made violence in the country a major issue in the campaign. Venezuela has one of the highest murder rates in the world, and is plagued by kidnappings and other crimes.

The two deaths on Saturday were the first to be attributed to campaign violence in this election, but there have been injuries in other confrontations involving shootings and rock throwing. It is common for Chávez supporters, often on motorcycles and sometimes carrying guns, to try to disrupt opposition events.

Mr. Chávez has repeatedly accused his opponents of planning acts of violence that would be carried out at the end of the campaign or in the aftermath of the election. But he also routinely uses aggressive language in his political speeches. At a rally in Guarenas, a city east of Caracas, on Saturday, he called Mr. Capriles a fascist and a liar and vowed to flatten him and give him a beating.

On Sunday, speaking at a large rally in the western state of Zulia, Mr. Chávez called the deaths regrettable, but did not say who was responsible.

"We will not permit the bourgeoisie to take Venezuela down the road of violence," he said. "And we should not resort to provocations. No violence."

At the Capriles rally in Caracas on Sunday, marchers said they thought the two killings would motivate voters. "It will give us more strength to get rid of him," said Jonathon Cabrera, 37.

There was no official estimate of the crowd's size on Sunday, but The Associated Press said it appeared to exceed 100,000.

María Eugenia Díaz contributed reporting.