* Doctors found malignant cells in his pelvic area
* New operation due to take place in Cuba within days
* Vice President Maduro tapped as possible successor
By Daniel Wallis and Andrew Cawthorne
CARACAS, Dec 9 (Reuters) - Venezuela's President Hugo Chavez returns to Cuba on Sunday for more surgery after a recurrence of cancer led him to name a successor for the first time in case the disease ends his 14-year dominance of the OPEC nation.
Supporters gathered in squares across the South American country, shocked and saddened by the news from the 58-year-old socialist leader, who made the announcement in a late-night broadcast on Saturday from the presidential palace.
In his first public acknowledgement that cancer may cut his tumultuous years in power, Chavez said Vice President Nicolas Maduro would take over if he is incapacitated, and urged supporters to vote for him if an election is held.
"It is absolutely necessary, absolutely essential, that I undergo a new surgical intervention," the president said in his speech, flanked by ashen-faced ministers. "With God's will, like on the previous occasions, we will come out victorious."
His departure from office, either before or after the scheduled Jan. 10 start of his new term, would trigger an election within 30 days. It would mark the end of an era for the Latin American left, depriving them of one of their most acerbic voices and Washington's main irritant in the region.
A clutch of Latin American and Caribbean neighbors, from Cuba and Nicaragua to Bolivia and Ecuador, depend on Chavez's oil-fuelled generosity to bolster their fragile economies.
An unruly transition from Chavez's highly centralized rule could also raise the specter of political instability in Venezuela, which holds the world's largest crude oil reserves.
Allies lack his famous charisma and may struggle to control his unwieldy coalition of military and leftist leaders.
FORMER BUS DRIVER NAMED SUCCESSOR
Among them, though, Maduro - a 50-year-old, mustachioed former bus driver and union leader - is widely viewed as the most popular among Venezuelans thanks to his affable manner, humble background and close relationship with Chavez.
Speculation about Chavez's health had grown during a three-week absence from public view that culminated in his latest trip for medical tests in Cuba - where he has undergone three cancer operations and had two tumors removed since June 2011.
He returned to Venezuela on Friday after those tests, and is due to have the operation in Cuba in the next few days. Venezuela's National Assembly was to hold a special session on Sunday to formally approve his trip.
Chavez said he had rejected the advice of his medical team to have the surgery sooner, on Friday or this weekend, telling them he needed to fly back to Venezuela to seek that permission.
"I decided to come, making an additional effort, in truth, because the pain is not insignificant," Chavez said in a televised address also shown live in Cuba.
The president's return to Cuba may mark the start of another lengthy period of silence from government officials, combined with furious rumors over what political changes might be in store and what Chavez's actual condition is.
He has never said what type of cancer he has, though when initially diagnosed in mid-2011 the government said Chavez's problem was in the pelvic area.
Opposition leaders wished Chavez well, but criticized him for excessive secrecy and not using local healthcare.
"A president should be treated in his country. We have the best equipment and doctors here. But we respect the fact he will be in Cuba," one leader, Julio Borges, told local media.
He criticized Chavez for declaring himself cured prior to the election campaign then admitting he had been unwell after winning: "We have the right to demand to be told the truth."
NEW PRESIDENTIAL VOTE?
The shock on the faces of Cabinet ministers during Saturday's late-night broadcast signaled uncertainty over who will hold the reins absent Chavez, even in the top echelons of power.
Chavez has been receiving treatment at Havana's Cimeq hospital as a guest of his close friend and political mentor, former Cuban leader Fidel Castro. He is guaranteed tight security and privacy on the communist-led Caribbean island.
The usually loquacious Venezuelan leader had sharply cut back appearances since winning the Oct. 7 election, saying the campaign and radiation therapy had left him exhausted.
Venezuela's constitution stipulates a new election if Chavez leaves office, unless it is in the last two years of his six-year term when his vice-president would take over.
Publicly naming long-time ally Maduro was a surprise.
"He is a complete revolutionary, a man of great experience despite his youth, with great dedication and capacity for work," Chavez said. "In a scenario where they were obliged to hold a new presidential election, you should choose Nicolas Maduro."
Maduro's trade union background appeals to Chavez's working-class supporters, while his years as foreign minister provided opportunities for networking abroad.
He may win less support from the military wing of the Socialist Party, which controls many top government posts.
The naming of Maduro sidelines Diosdado Cabello, a Congress head and former military comrade of Chavez. Perhaps fearing in-fighting, Chavez urged "unity" again-and-again in his comments.
"I never argue with Chavez's instructions, I obey them," Cabello said shortly afterwards. "I am at the service of the vice-president, at the service of the fatherland."
The opposition could find itself in its best position to oust his administration since Chavez took power in 1999. Many voters have ignored the failings of Chavez's government because of their intense emotional connection to him.
Any new poll would potentially give just-defeated presidential candidate Henrique Capriles another chance. He won 44 percent and a record 6.5 million votes for the opposition.
Though past polls have shown Capriles is more popular than any of Chavez's allies, including Maduro, the vice president will benefit from his boss's personal blessing.
Venezuela's widely traded bonds are likely to soar when markets open on Monday on expectations that Chavez's renewed illness will pave the way for a more market-friendly government.
Chavez's cancer saga has once again distracted attention from major national issues like state elections in a week, a possible devaluation of the local bolivar currency, and a proposed amnesty for jailed and exiled political foes.
Messages of support for Chavez poured in from friends and sympathizers round the region, including Colombia's FARC rebels.