* Doctors found malignant cells in his pelvic area
* Surgery to take place in Havana within days
* Taps Vice President Maduro as possible successor
By Daniel Wallis and Marianna Parraga
CARACAS, Dec 8 (Reuters) - Venezuela's Hugo Chavez said on Saturday that his cancer had returned and he would undergo another operation in the coming days, and for the first time the president named a successor if anything happened to him.
The news was a big blow for supporters of the 58-year-old socialist leader, who elected him in October to a new six-year term in office. Twice since mid-2011 Chavez has said he was cured, and then had to have more surgery.
In an emotional television broadcast from the Miraflores presidential palace, Chavez was flanked by ministers and looked resolute. He even sang, briefly. And in his first comments on a possible successor, he said supporters should vote for Vice President Nicolas Maduro.
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 since June 2011. He returned to Venezuela on Friday.
"Unfortunately, during these exhaustive exams they found some malignant cells in the same (pelvic) area ... . It is absolutely necessary, absolutely essential, that I undergo a new surgical intervention," the president said.
"With God's will, like on the previous occasions, we will come out of this victorious. I have complete faith in that."
Chavez, who has dominated Venezuelan politics since taking power 14 years ago, said he would return to Cuba on Sunday, and that the operation would take place there in next few days.
He said he had rejected the advice of his doctors to have the surgery sooner, on Friday or this weekend, telling them he needed to fly back to Venezuela to seek the permission of lawmakers to return for the operation.
"I decided to come, making an additional effort, in truth, because the pain is not insignificant," Chavez said. "But with treatment and painkillers, we are in the pre-operation phase."
MADURO GETS THE NOD
Chavez has been receiving treatment at the tightly guarded Cimeq hospital in Havana as a guest of his friend and political mentor, former Cuban leader Fidel Castro.
The normally garrulous president had sharply cut back his public appearances since winning the Oct. 7 election, saying the campaign and radiation therapy had left him exhausted.
Under Venezuela's constitution, an election would have to be held within 30 days if Chavez were to leave office in the first four years of his next term, which is due to begin on Jan. 10.
For the first time, in a rare admission that he might not be able to govern for as long as he hopes, he singled out Maduro - a 50-year-old former bus driver and union leader - as his chosen candidate.
"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."
In addition to putting his own future in doubt, the news that Chavez's cancer has returned is also a blow to ruling Socialist Party candidates who wanted him to campaign alongside them before elections for state governors on Dec. 16.
Another prolonged absence recuperating in Cuba could also postpone important policy decisions, such as a widely expected devaluation of the bolivar currency.