UPDATE 3-Venezuela's Chavez suffers cancer again, names potential heir

* 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.

Throngs of shocked supporters gathered in squares across the

South American country to pray for and show solidarity with the

58-year-old socialist leader, who was re-elected as president in

October.

In his first public acknowledgement that cancer could put an

end to his tumultuous rule, 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.

"With God's will, like on the previous occasions, we will

come out victorious," Chavez said late on Saturday from the

presidential palace next to ashen-faced ministers.

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-financed generosity to bolster their fragile economies.

An unruly transition from Chavez's highly centralized rule

also could raise the specter of political instability in

Venezuela, which holds the world's largest crude oil reserves.

Allies lack Chavez's 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. He has undergone three

cancer operations and had two tumors removed there since June

2011.

He had twice claimed to be cured, only for the cancer to

return.

Chavez arrived in Venezuela on Friday after the latest

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 revealed what type of cancer he has, saying

only that it was in the pelvic area. He said on Saturday that

the latest recurrence was in the same region.

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," Borges

said.

NEW PRESIDENTIAL VOTE?

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.

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, who heads

Congress and is a 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 afterward. "I am at the service of the vice

president, at the service of the fatherland."

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 ignored the failings of Chavez's government because

of their intense emotional connection to him.

Henrique Capriles, a state governor, lost to Chavez in the

October election, winning 44 percent support and a record 6.5

million votes for the opposition. He has broad support in

opposition ranks and could run for president again.

Although 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 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 for Chavez poured in from friends and

sympathizers round the region, including Colombia's FARC rebels.

"Onward, my president. I love you, we need you, not only us

but many other countries," said housewife Gladys Millan, 45,

weeping as she stood with other Chavez supporters in Caracas'

Bolivar Square for a prayer meeting.

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