By Andrew Cawthorne and Daniel Wallis
CARACAS (Reuters) - Venezuela's Hugo Chavez underwent surgery in Cuba on Tuesday for a cancer recurrence that has thrown his presidency into jeopardy and upended politics in the South American OPEC nation.
"My dear friend and colleague, Comandante Hugo Chavez, is going through the toughest times of his life," said Ecuadorean President Rafael Correa, a fellow leftist. "He is being operated on right now. It's a very delicate operation."
The 58-year-old Chavez's surgery in Cuba, a close ally, was his fourth since mid-2011. Doctors found malignant cells again in his pelvic area soon after he won re-election in October, leading him to name a successor in case he has to step down.
Two Venezuelan government sources said the operation had been completed by Tuesday evening, but there was no immediate word from official spokesmen. The information minister had earlier said the president's medical team was optimistic.
State TV ran footage of prayer meetings held around the country in solidarity with Chavez, and of ministers pledging their loyalty to the socialist leader. Details of his condition are treated almost as a state secret.
Having declared himself cured twice in the past, Chavez now worries the latest recurrence could end his tumultuous 14-year rule. But he retains hope of recovering in time for the January 10 start of his new six-year term in office.
Chavez named his vice president and foreign minister, Nicolas Maduro, as a potential heir to lead his self-styled revolution in a nation of 29 million people with the world's largest oil reserves.
Maduro, 50, a former bus driver and union activist, lacks his boss's charisma and political flair but would represent policy continuity should he take over. He has already taken control of day-to-day government business.
'NOT A MONARCHY'
The naming of Maduro has irked some in Venezuela's opposition, who say voters - not Chavez - will decide who follows him if an election is held within 30 days of his leaving office, as required under the constitution.
"Venezuela is not a monarchy with a prince as heir," said one opposition leader, Antonio Ledezma.
Should an election be held, opposition flag bearer Henrique Capriles, who lost to Chavez in the October presidential ballot but scored a record 6.5 million votes for the opposition, could have a second crack at power.
In a newspaper interview on Tuesday, Capriles declined to speculate on a possible new presidential bid and repeated his best wishes to Chavez for recovery. But he criticized the secrecy surrounding the president's treatment.
"Venezuelans have the right to know," he said.
Chavez was assumed to be a patient once more at Havana's Cimeq hospital, though no official information on the situation was coming out of the communist-run Caribbean island. Cancer experts said the recurrence of his disease was bad news.
"The chances for long-term recovery are highest the first time around, when you attack a cancer with surgery and chemotherapy. Whenever a cancer recurs, your chances for a long-term positive outcome decrease," said Dr. Axel Grothey, an oncologist at Mayo Clinic in Rochester, Minnesota.
Grothey has not been involved in the president's treatment but said he had seen many patients in a similar situation.
The stakes are high for Cuba and other socialist-run nations around Latin America and the Caribbean which depend on the Chavez government for subsidized oil and other economic aid.
Messages of support have poured in from abroad.
"He changed the history of Venezuela and of a large part of Latin America," said Correa, a central member of the Chavez-led Alba bloc of leftist nations in the region.
"He's in good spirits. You know what Hugo's like, always ready for tough battles with optimism and faith ... I'm not going to lie, we are very worried. It is a serious matter."
U.S. actor Sean Penn, one of Chavez's most prominent international supporters, joined a vigil in Bolivia. "He's one of the most impressive forces on the planet and we need to show him our love," Penn was quoted as saying by local media.
Chavez's health woes had sparked a rally in Venezuela bonds, given many investors' preference for a more business-friendly government in Caracas. Gains were trimmed slightly on Tuesday.
Western investors have increasingly shunned Venezuela under Chavez - especially given his nationalizations of large swathes of the economy - giving companies from China, Russia, Iran, Belarus and other allies a chance to grab footholds.
In London, veteran emerging market investor Mark Mobius said any change to a more market-friendly government in Venezuela would encourage him to invest there once more.
"Regime change has probably entered into a countdown phase," said Siobhan Morden of New York-based Jefferies investment bank.
The health saga has once again eclipsed major national issues such as state elections on Sunday, a widely expected devaluation of the bolivar currency and a proposed amnesty for Chavez's jailed and exiled political foes.
"The nation is paralyzed," said Jorge Botti, head of the main private business group, Fedecamaras, urging the government to boost sales of dollars through its currency-control system.
The lack of greenbacks has left merchants struggling to import products and pushed the black market rate for the bolivar currency to four times its official rate of 4.3.
Venezuelans were expecting a currency devaluation around the New Year, though Chavez's illness has put this in doubt.
Maduro is a committed socialist who has indicated he would follow Chavez's policies, including heavy state control of the economy. "Currency controls have worked well, and yes, they can be improved. They are going to be improved," he said on Monday.
(Additional reporting by Caracas bureau, Eduardo Garcia in Quito, and Julie Steenhuysen in Chicago; Editing by Kieran Murray and Christopher Wilson)