By Andrew Cawthorne and Jeff Franks
CARACAS/HAVANA (Reuters) - Venezuelan President Hugo Chavez flew into Cuba on Wednesday for cancer-linked medical treatment that revived questions about the viability of his socialist rule and left Venezuelans again guessing about his exact condition.
After weeks of scarce public appearances, Chavez, 58, announced in a letter on Tuesday that he was going to Havana for therapy known as "hyperbaric oxygenation" - a method used to reduce bone decay caused by radiation therapy.
Communist Party daily Granma confirmed his arrival in Cuba.
In Havana, Chavez enjoys the friendship of past and present Cuban leaders Fidel and Raul Castro, plus guaranteed privacy on the tightly controlled Caribbean island.
Venezuelans, who have been endlessly speculating about Chavez's cancer since it was discovered in mid-2011, were not sure what to make of the latest twist - debating whether it was normal post-radiation treatment or a serious downturn.
"I really don't know what he has," Chavez's cousin, Guillermo Frias, told Reuters from the president's rural hometown state Barinas. "But anyway, I always pray for him every night. I stop at a shrine on the corner and always remember him.
"I hope he recovers fine. I'm sure he will. The election campaign was tough for him. He went too far."
Though he had declared himself cured, Chavez appeared exhausted at the end of his successful presidential re-election bid in October. He later admitted radiation had taken its toll.
The normally garrulous and omnipresent leader has made only a few, relatively short public appearances, mainly on state TV, at his presidential palace since his October 7 win. One opposition newspaper dubbed him "The Invisible Man".
Unlike multiple past trips to Cuba, during treatment for three operations on two tumors in his pelvic area, state TV did not show images of Chavez departing or arriving this time.
Chavez has an open-ended authorization from Congress to travel, but aims to be back at least for the January 10 start of his new term, if not for a couple of regional summits before.
His absence leaves newly appointed Vice President Nicolas Maduro, 49 - a former bus driver and union leader - in a prominent position amid speculation among Venezuelans over who could replace Chavez should he leave power.
Congress head Diosdado Cabello, a former military comrade of Chavez's, also is frequently touted as a possible successor to lead the ruling Socialist Party. Under the constitution, an election would have to be held if Chavez leaves office within the first four years of his new six-year term.
Chavez's return to Cuba overshadowed the buildup to state elections on December 16, where the opposition aims to overcome disappointment at the presidential loss to make inroads.
A prolonged absence could potentially postpone major policy decisions, such as a widely expected devaluation of the bolivar currency after heavy pre-election state spending.
After removal of a first cancerous tumor, Chavez wrongly declared himself cured in late 2011. He again pronounced himself cancer-free in mid-2012 after removal of a second tumor.
The hyperbaric oxygenation therapy, or HBOT, he was due to receive involves breathing pure oxygen in a pressurized chamber.
In addition to the bone-weakening side effects of radiation on cancer sufferers, experts say HBOT is used to treat conditions including infections, abscesses and decompression sickness - or the "bends" - that can afflict deep sea divers.
Nelson Bocaranda, a prominent pro-opposition journalist, said Chavez had been suffering intense pain in his bones and waist area of late, forcing him to rest and take pain-killers.
In his widely read "Rumors" column on Wednesday, Bocaranda published a supposed medical report from Havana's Cimeq hospital, with a relatively uninflammatory diagnosis.
"It's a matter of giving him therapy for pain and stabilization so he has a better quality of life," said the report, which could not be confirmed.
"His physical state is normal; loss of weight reasonable; high tension constant; abdominal nausea and pains; good emotional state but with variable depression; tolerable pain thresholds and reaction to treatment applied. He's rested in recent days and had little pressure from government functions."
Venezuelan officials, who frequently decry Bocaranda as a gossip and liar, gave no details of Chavez's health. One medical source with knowledge of Chavez's treatment said the HBOT may last several months and was a common "palliative treatment."
An opposition leader, Henry Ramos Allup, demanded transparency from the government. "The president's illness is a matter of state. The country has the right to know how exactly how serious it is," he said.
Given investor hopes for a more market-friendly government, Venezuela's widely traded bonds not surprisingly have risen.
On the streets, there was both solidarity and skepticism concerning Chavez.
"He exploited this thing of going to Cuba during the election campaign," motorbike taxi driver Omar Rivas, 55, said, noting the wave of sympathy Chavez received in the past.
"That man doesn't have anything. He was never sick."
Teacher Ana Maria Garcia, 26, had a kinder reading.
"I don't understand what he has, but I hope he recovers quickly. He's a winner," she said.
(Additional reporting by Mario Naranjo and Diego Ore; Editing by Bill Trott)