Venezuela's opposition presidential candidate Capriles greets supporters during a campaign rally in Valencia.

Two influential supporters of Venezuela's opposition presidential candidate Henrique Capriles have been shot dead by gunmen during a campaign rally ahead of the presidential election.

One of the victims, Geison Valero, belonged to Capriles' party Primero Justicia (First Justice) and the other, Omar Fernandez, was an independent.

A suspect has been detained, said the justice ministry.

The victims were on their way to a planned rally in Barinas, the home state of Venezuelan President Hugo Chavez.

The two men were trying to gain access to a road that had been blocked by Chavez supporters. As the duo left their car, they were shot dead by gunmen in a van.

Witnesses said the van belonged to the state oil company PDVSA, according to a Reuters report.

"I'm so sad at this bad news," Capriles tweeted following the incident.

"This tragedy gives us more strength and faith to fight for a Venezuela where justice and non-violence reign," said Primero Justicia in a statement.

The Democratic Unity coalition, a union of Venezuela's opposition parties, demanded an immediate investigation into the deaths. Nearly 30 opposition parties support Capriles against the 58-year-old leftist president Chavez.

Chavez announced an investigation into the incident and vowed to bring the culprits to justice. Chavez is seeking re-election in the forthcoming national poll.

The campaign trail has been littered with reported clashes between supporters of Chavez and Capriles.

There is widespread concern that further violence may erupt as the closely fought election reaches its final stages.