The shooting took place in the central Venezuela town of Altagracia de Orituco yesterday, the leader of Democratic Action party, Carolos Prosperi, said.
He said he heard gunshots as the rally was breaking up and the party's leader for the town, Luis Manuel Diaz, was hit.
Another top party official, Henry Ramos Allup, said Diaz was killed while sitting on stage next to Lilian Tintori, wife of jailed opposition leader Leopoldo Lopez, who is one of the best known faces of forces opposing Venezuela's socialist government.
The opposition says several activists have been wounded in recent days as government supporters crashed campaign events.
The socialist party has denied that it is trying to intimidate the opposition with violence and called the allegations nothing more than the latest craze among the government's foes.