* Victims belonged to parties behind opposition's Capriles
* Venezuelans transfixed by close race, fret about trouble
* Chavez, Capriles hit streets with one week to go
By Andrew Cawthorne and Deisy Buitrago
CARACAS, Sept 29 Gunmen shot and killed two local leaders of parties backing presidential challenger Henrique Capriles on Saturday in the worst violence of a volatile campaign before Venezuela's election next weekend.
Capriles' party, Primero Justicia (First Justice), said the gunmen fired from a van that witnesses identified as belonging to state oil company PDVSA or the local mayor's office during a rally in the agricultural state of Barinas.
The government of President Hugo Chavez, who is seeking re-election, confirmed the deaths and vowed the perpetrators would be brought to justice. Interior Minister Tareck El Aissami said the circumstances of the attack were still under investigation.
Venezuela is awash with guns, and violent crime is frequently cited as voters' No. 1 concern.
There had been shootings and fistfights in previous opposition rallies as "Chavistas" and Capriles supporters clashed, but no deaths.
"This tragedy gives us more strength and faith to fight for a Venezuela where justice and non-violence reign," said Primero Justicia, the party of one of the victims.
Another two people were injured, and there were six arrests after the attack on an opposition motorcade that had been blocked by Chavez supporters, Primero Justicia said in an account not confirmed by police or other authorities.
"I'm so sad at this bad news," Capriles said via Twitter. The opposition Democratic Unity coalition, which has united Venezuela's opposition parties, demanded a quick investigation.
Aissami said police were doing just that. "It was an isolated incident," he told state TV.
On the campaign trail, Chavez showed off new infrastructure projects in Caracas, while Capriles accused him of wasting Venezuela's money on foreign allies.
With polls inconclusive, both men are wooing undecided voters in what looks likely to be the tightest presidential election of the charismatic socialist leader's 14-year rule.
Despite two bouts of cancer since mid-2011, Chavez, 58, has declared himself completely cured and is trying to recapture some of his old panache and energy to win a new six-year term.
On Saturday, he inaugurated a monorail, then inspected extensions to the subway system, and a cable car in poor areas of Caracas typical of his power base.
The projects cost a combined $2.5 billion.
"We are not thinking about making money. That's the difference with capitalism," Chavez said in Petare, one of the largest slums in Latin America.
"The loser will have to go to the moon and see if he can govern a rock there because here the bourgeoisie are never coming back," Chavez quipped of Capriles, whom he portrays as representing a heartless, right-wing elite.
Later, in Guarenas town outside Caracas, the president drove through crowds in an open vehicle dubbed by some the "Chavez-mobile." He sang, danced and gave an exuberant speech in a show of energy few would have expected just months ago when he was publicly praying to be saved from cancer.
Capriles, a 40-year-old state governor who has a centrist political vision and sees Brazil's mix of free-market economics and strong welfare polices as his model, has been crisscrossing Venezuela all year in an exhausting campaign.
Addressing thousands in Falcon state, in west Venezuela, Capriles accused Chavez of making false promises to the public, while squandering oil revenues on foreign allies.
"The government prefers to build a refinery in Nicaragua, or send oil and worry about power cuts in Cuba, but it doesn't care about blackouts here in Falcon," he said.
Of the six or so best-known pollsters in Venezuela, a majority put Chavez ahead, but they also show Capriles creeping up in recent weeks, and two put him just ahead.
Venezuelans are transfixed by the race, but also nervous of possible violence if the result is close and disputed.
Foreign investors hope the more business-friendly Capriles will take over and end a nationalization drive and other radical policies that have polarized Venezuela like never before and made Chavez one of the world's most controversial leaders.
Chavez promises to "deepen" socialism if he wins. That will likely mean continued spending on his popular welfare "missions," new confrontations with the private sector, and more support for his leftist allies in the region.
Opposition leaders are angry at Chavez's use of state resources in his campaign, but say the electronic-based vote system should be hard to rig on election day, since they will have their own observers at most voting booths.
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*(Additional reporting by Brian Ellsworth, Deisy Buitrago; Editing by Peter Cooney)