* Cancer threatening to end his 14-year-old rule
* Officials say Chavez is "fighting" for his life (Updates with Maduro quotes, details)
By Andrew Cawthorne and Daniel Wallis
CARACAS, March 1 (Reuters) - Senior aides and relatives of Venezuela's Hugo Chavez countered on Friday a crescendo of rumors that the socialist president may be dead from cancer, saying he was still battling for his life.
"There he is, continuing his fight, his battle, and we are sure of victory!" his older brother Adan Chavez, the governor of Barinas state, told cheering supporters.
Speculation about Chavez, 58, hit fever pitch this week, fed in part by assertions from Panama's former ambassador to the Organization of American States (OAS), Guillermo Cochez, that the Venezuelan leader had died.
"The launching of absurd and bizarre rumors by the right wing simply discredits them and isolates them further from the people," Chavez's son-in-law Jorge Arreaza, who is also the country's science minister, said on Twitter.
Apart from one set of photos showing Chavez lying in a hospital bed, he has not been seen nor heard from in public since Dec. 11 cancer surgery in Cuba, his fourth such operation.
The president made a surprise pre-dawn return to a military hospital in Caracas last week, with none of the fanfare that had accompanied his previous homecomings after treatment.
Vice President Nicolas Maduro, the OPEC nation's de facto leader and Chavez's preferred successor, said several times this week that the president was fighting for life and urged Venezuelans to be patient.
Opposition politicians accuse the government of being deceitful about Chavez's condition, and compare the secrecy over his medical details with the transparency shown by other Latin American leaders who have suffered cancer.
Cochez said Chavez's relatives had switched off his life support several days ago after he had been in a vegetative state since the end of December. He challenged Venezuela's government to prove him wrong by showing the president in public.
Across the South American nation of 29 million people, Venezuelans are extremely anxious, speculating almost non-stop about Chavez's condition and wondering what the potential end of his 14-year rule might mean for them.
Adding to the tension, several dozen opposition-supporting students have chained themselves together in a Caracas street, demanding to see the president and arguing that Maduro has no right to rule because he was not elected.
With everyone on edge, the relatively routine shooting by police of a murder suspect during a gun battle in downtown Caracas on Friday forced Information Minister Ernesto Villegas to take to Twitter to issue reassurances.
"(Some people) took advantage of the episode to try to sow panic in the city center," he said. "The situation is calm."
Should Chavez die or step down, an election would be held within 30 days, probably pitting Maduro against opposition leader and state governor Henrique Capriles for leadership of the country which boasts the world's biggest oil reserves.
The stakes are high for the region, too, given Chavez's role as the most vocal critic of Washington in Latin America and with the aid he has given leftist governments from Cuba to Bolivia.
Amid the flurry of rumors, Spain's ABC newspaper said on Friday that Chavez had been taken to a presidential retreat on La Orchila island in the Caribbean off Venezuela's coast with his closest family to face the "final stages" of his cancer.
Venezuelan officials have frequently lambasted ABC as being part of an "ultra-right" conspiracy spreading lies about Chavez.
"The bourgeoisie harass him and they assault him constantly," Maduro said on Friday. "Stop the attacks on the commander! Stop the rumors, stop trying to create instability!"
In the latest of a series of short updates on Chavez's health, the government said last week that his breathing difficulties had worsened, and he was using a tracheal tube.
Officials say he suffered a severe respiratory infection following the six-hour operation he had in December for a cancer that was first detected in his pelvic region in June 2011.
Chavez has never said what type of cancer he has.
Remarkably, two opinion polls this week showed that a majority of Venezuelans - 60 percent in one survey, 57 percent in another - believe he will be cured.
Chavez's millions of passionate supporters, who love his down-to-earth style and heavy spending of oil revenue on welfare policies, are struggling to imagine Venezuela without him.
"Of course, he's coming back, back to government," said Jose Urbina, 47, buying photos of Chavez at a pro-government rally. "I want to remember him. I want to put them in my house." (Additional reporting by Girish Gupta; Editing by Vicki Allen)