* Chavez makes show of strength after cancer relapse
* Supporters flood Caracas to back Chavez re-election bid
* Will face united opposition challenger Capriles on Oct. 7 (Adds Chavez quotes, details)
By Brian Ellsworth
CARACAS, June 11 (Reuters) - Venezuelan President Hugo Chavez led his biggest rally since he was diagnosed with cancer a year ago, seeking to stage a show of strength on Monday as he heads into a re-election campaign to extend his 13 years of socialist rule.
The former soldier delivered a nearly three-hour speech with his practiced mix of folkloric spontaneity and militant discourse, marking a dramatic re-entry into the public eye after months of keeping a low profile during cancer treatment.
The event was all the more important after his opposition rival, Henrique Capriles, put on a show of youthful vigor on Sunday by leading a 10-km (6.2-mile) march with hundreds of thousands of supporters.
“We have been facing the psychological warfare of the adversary, that Chavez has only a few days left to live, that he is in a wheelchair,” Chavez told a pulsing crowd after filing his re-election bid with the authorities.
“Now, here I am once again in front of you, registering my candidacy in the name of the fatherland,” he said, wearing his trademark red beret and track-suit top in the colors of the Venezuelan flag.
The speech, which ranged from arcane Venezuelan historical references to casual banter with government ministers, marks the de facto launch of his campaign for the Oct. 7 vote.
He has steadily stepped up his public appearances in recent weeks after mostly communicating via Twitter or phone calls to state television.
Supporters flooded the streets of downtown Caracas in the hours prior to Chavez’s appearance, dancing to music pounding from speakers as giant inflatable Chavez dolls waved their arms above the crowd. Hundreds of buses that ferried his followers to Caracas stood parked in side streets.
Despite his vigor at the podium, Chavez’s face appeared unusually swollen and at one point he appeared to walk with some difficulty, prompting a scathing Twitter response from Capriles: “This candidate isn’t walking, he is out of gasoline!”
Chavez, in turn, tarred Capriles as a candidate “who smells of nothing, tastes of nothing, is completely insipid.” He joked that he would ask a famous Venezuelan comedian who briefly ran for president in 2006 to join the race because the opposition “doesn’t entertain anyone.”
He did not provide details on his condition or say if he would need more treatment.
Last year he had two operations to remove a baseball-sized tumor, and this year had a third operation only months after having declared himself “cancer free.”
Any turn for the worse in Chavez’s health could mean the end for his movement. That would be a blow to global leftist leaders who see him as an inspiration, but a boon to investors seeking free market reforms in Venezuela and oil companies keen on tapping the world’s biggest crude reserves.
Polls show two-thirds of Venezuelans believe he will get better.
Most of the country’s main pollsters show Chavez holding a double-digit lead over Capriles, as the combination of his ebullient charisma and a flood of public spending have offset his unusual silence caused by the cancer treatment.
“We’re just warming up the motors, while those losers sometimes seem like they’re already stalling,” Chavez said.
Since last year he has launched a wave of improvised social programs including pension funds for senior citizens and cash stipends for poor mothers, while giving away thousands of apartments to families that lost homes in floods.
Capriles’ campaign disputes the poll results, saying the effects of his nationwide house-to-house tour are not yet visible. They point out that Venezuelan public opinion is known to shift dramatically, as it did when Chavez came from behind in 1998 to win his first election.
Chavez supporters sent Twitter messages with the hash tag #VoyconChavez (#I’mgoingwithChavez). Adversaries responded by filling Venezuela’s notoriously vitriolic Twitter-sphere with messages tagged to #13añosdementiras (#13yearsoflies).
“Look at this sea of people; look at the happiness,” said Esther Martinez, a 66-year-old retiree dancing in a city square. “For every person that came out yesterday, we’ve brought out 10, 20, 30 more. And that’s going to be reflected in the election.”
Critics accuse Chavez allies of using state resources to swell demonstrations and forcing government employees to attend.
Opposition leader Leopoldo Lopez said the ruling Socialist Party had ordered ministries to help bring 120,000 people to the march, citing what he called an internal party document. (Additional reporting by Andrew Cawthorne, Deisy Buitrago and Mario Naranjo; Editing by Daniel Wallis, Christopher Wilson and Lisa Shumaker)