* 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
CARACAS, Dec 9 (Reuters) - Venezuelan 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.
Singing, praying and waving Chavez election posters from just two months ago, throngs of red-clad supporters gathered in squares across the South American country to show solidarity with the 58-year-old socialist leader.
In his first public acknowledgement that he might have to step down, Chavez said his vice president and foreign minister, Nicolas Maduro, would take over if he were incapacitated.
He urged supporters to back Maduro if there was a new vote.
"I have absolute confidence in the bright future that lies ahead for our beloved fatherland and the Bolivarian revolution," Chavez said in a short letter read to an emotional session of the National Assembly that approved his return to Havana.
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 also mark the end of an era for the Latin American left, depriving it of one of its most acerbic voices and the region's loudest critic of Washington.
Chavez's controversial rule has turned Venezuela into a deeply polarized nation, and much more so this year because of the uncertainty over his future, and the bruising election race.
"Go with God, my president, and come back fit and well," said Rosaria Villareal, a 41-year-old woman who joined the crowds in Caracas's Plaza Bolivar next to a statue of Chavez's idol and Venezuela's independence hero Simon Bolivar.
"Illnesses can be cured. What can't be cured is the opposition's damaged souls."
Chavez's health has big regional implications: a clutch of Latin American and Caribbean neighbors, from Cuba and Nicaragua to Bolivia and Ecuador, have come to depend on his oil-fueled largesse to bolster their fragile economies.
Cuba has been the biggest beneficiary, with Venezuela's government shipping about 115,000 barrels of oil a day to the communist-led island on preferential terms.
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 reserves.
FORMER BUS DRIVER NAMED SUCCESSOR
Among many outpourings of emotion from Chavez supporters around the nation of 29 million people, shamanic tribal leaders in headdresses shook bundles of leaves and chanted at an event in Amazonas state, in the jungle near the border with Brazil.
Lacking his remarkable charisma and political acumen, Chavez's allies could struggle to control his unwieldy coalition of military and leftist leaders.
Among them, however, Maduro - a 50-year-old former bus driver and union leader - is widely viewed as the most popular, thanks to his affable manner and close relationship with Chavez.
While his humble background appeals to the president's working class supporters, Maduro's six years as foreign minister have also boosted his profile with the leaders of China, Russia and other world powers.
He has an easygoing style but is a firm believer in Chavez's leftist policies and has often led fierce criticism of the United States. Maduro's wife is also a powerful figure in the ruling Socialist Party. Like Maduro, she was once a leader of the National Assembly, and she is now the attorney general.
In a national broadcast late on Saturday from the presidential palace, Chavez told ashen-faced ministers that he would have to return to Cuba for another operation.
He praised Maduro as a "complete revolutionary" - but the vice president could struggle to win support from the Socialist Party's military wing, which controls many top government posts.
The surprise naming of Maduro sidelines Diosdado Cabello, who heads Congress and is a former army comrade of Chavez. Perhaps fearing in-fighting, Chavez repeatedly called for unity.
Speculation about his health had grown during a three-week absence from public view that culminated in his latest trip for medical tests in Cuba. He has undergone three cancer operations and had two tumors removed there since June 2011.
He twice claimed to be cured, only for the cancer to return.
SILENCE AND RUMORS
If a new election were needed, the opposition could be in its best position to win since Chavez took power in 1999. Many voters have overlooked the government's failings because of their deep emotional connection with the president.
Henrique Capriles, a state governor, lost to Chavez in the October, but he received 44 percent of ballots cast, a record 6.5 million votes for the opposition. Previous polls showed him beating any of Chavez's allies including Maduro, he retains broad support in the opposition, and he could run again.
Capriles wished Chavez a speedy recovery on Sunday, but said Venezuelans were surprised because the president had declared himself completely cured before the Oct. 7 vote.
He said the people would decide on a replacement in a new election - if it came to that. "There is no provision for succession in the country. This is not Cuba, nor is it a monarchy where there is a king and he designates a successor."
In the National Assembly, opposition lawmakers were heckled by the president's supporters as they wished Chavez well - but then called for more transparency about his health. He has never said what type of cancer he has, only that it was in his pelvic area. The next operation will take place in days.
During the turbulent session, several Socialist Party legislators gave fiery speeches backing the president, their voices breaking, while some hugged. Others called for the people to be alert, to unite and to be on guard against conspiracies.
"Comandante, go to Cuba in peace. The people love you," Cabello said after the assembly voted to approve Chavez's trip - a formality each time he travels abroad for more than five days.
Looking grim, he accused the opposition of trying to benefit from Chavez's illness. "I am very sad. I'm hurting. But don't confuse sadness for weakness. Don't get it wrong," he cautioned.
Venezuela's widely traded bonds are likely to soar when markets open on Monday on bets that Chavez's renewed illness will lead to 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 bolivar currency, and a proposed amnesty for jailed and exiled political foes.
Messages of support poured in from well-wishers around the region, including Colombia's Marxist FARC rebels.