Chavez acknowledged the seriousness of his health situation in a televised address Saturday night, saying for the first time that if he suffers complications Vice President Nicolas Maduro should be elected as Venezuela's leader to continue his socialist movement.
The president is scheduled to be sworn in for a new six-year term Jan. 10.
"There are risks. Who can deny it?" Chavez said, seated at the presidential palace beside Maduro and other aides. "In any circumstance, we should guarantee the advance of the Bolivarian Revolution."
Chavez, who won re-election on Oct. 7, said he would undergo the operation in Havana in the coming days. He asked for lawmakers to grant permission for him to fly to Havana on Sunday, but it was unlikely that his request would be denied. It was unclear what time he would leave.
Under the Venezuelan constitution, as vice president Maduro would automatically fill in as president on a temporary basis should Chavez be unable to finish the current term concluding in early January.
But the constitution also says that if a president-elect dies before taking office, a new election should be held within 30 days. In the meantime, the president of the National Assembly is to be in charge of the government.
Several outside medical experts said that based on Chavez's account of his condition and his treatment so far, they doubt the cancer can be cured.
Chavez said he hasn't given up.
"I hope to give you all good news in the coming days," said Chavez, who held up a crucifix and kissed it. "With the grace of God, we'll come out victorious."
More than 1,000 supporters gathered Sunday in Plaza Bolivar in downtown Caracas to show solidarity for the president, many wearing the red T-shirts of his movement and chanting: "Ooh-Ah! Chavez isn't going away!"
The president, who just returned from Cuba early Friday, told television viewers late Saturday that tests had found a return of "some malignant cells" in the same area where tumors were previously removed.
Chavez's quick trip home appeared aimed at sending a clear directive to his inner circle that Maduro is his chosen successor. He called for his allies to pull together, saying: "Unity, unity, unity."
He also said it's important for the military to remain united, saying: "the enemies of the country don't rest."
Chavez said his doctors had recommended he have the surgery right away, but that he had told them he wanted to return to Venezuela first.
"I want to go there. I need to go to Venezuela," Chavez recalled telling his doctors. "And what I came for was this," he said, seated below a portrait of independence hero Simon Bolivar, the inspiration of his Bolivarian Revolution movement.
Chavez had named Maduro, his longtime foreign minister, as his choice for vice president three days after winning re-election. Maduro, a burly former bus driver, has shown unflagging loyalty and become a leading spokesman for Venezuela's socialist leader in recent years.
Chavez said that if new elections are eventually held, his movement's candidate should be Maduro.
"In that scenario, which under the constitution would require presidential elections to be held again, you all elect Nicolas Maduro as president," Chavez said. "I ask that of you from my heart."
"He's one of the young leaders with the greatest ability to continue, if I'm unable to ... continue with his firm hand, with his gaze, with his heart of a man of the people," Chavez said, also saying that Maduro's leadership and "the international recognition he has earned" make him fit to become president.
Chavez was flanked by both Maduro and National Assembly President Diosdado Cabello and held a small blue copy of the constitution in his hands.
Announcements on state television called for Chavez's supporters to gather in city plazas throughout the country on Sunday. At a plaza in downtown Caracas, some expressed optimism that Chavez would pull through it. Others said they weren't sure.
"I love Chavez, and I'm worried," said Leonardo Chirinos, a construction worker. "We don't know what's going to happen, but I trust that the revolution is going to continue on, no matter what happens."
Chavez called his relapse a "new battle." It will be his third operation to remove cancerous tissue in about a year and a half.
The 58-year-old president first underwent surgery for an unspecified type of pelvic cancer in Cuba in June 2011, after an operation for a pelvic abscess earlier in the month found the cancer. He had another cancer surgery last February after a tumor appeared in the same area. He has also undergone chemotherapy and radiation treatments.
Chavez said tests immediately after his re-election win had shown no sign of cancer. But he said he had swelling and pain, which he thought was due to "the effort of the campaign and the radiation therapy treatment."
"It's a very sensitive area, so we started to pay a lot of attention to that," he said, adding that he had reduced his public appearances.
Chavez made his most recent trip to Cuba on the night of Nov. 27, saying he would receive hyperbaric oxygen treatment. Such treatment is regularly used to help heal tissues damaged by radiation treatment.
Chavez said that while he was in Cuba tests detected the recurrence of cancer.
He arrived back in Caracas on Friday after 10 days of medical treatment, but until Saturday night had not referred to his health. His unexplained decision to skip a summit of regional leaders in Brazil on Friday had raised suspicions among many Venezuelans that his health had taken a turn for the worse.
Dr. Carlos Castro, scientific director of the League Against Cancer in neighboring Colombia, told The Associated Press that he expects the operation will likely be followed by more chemotherapy.
"It's behaving like a sarcoma, and sarcoma doesn't forgive," Castro said, adding that he wouldn't be surprised if the cancer had also spread to the lungs or other areas.
"We knew this was going to happen," he said. "This isn't good."
Throughout his treatment, Chavez has kept secret various details about his illness, including the precise location of the tumors and the type of cancer. He has said he travels to Cuba for treatment because his cancer was diagnosed by doctors there.
Dr. Julian Molina, an oncologist at the Mayo Clinic Comprehensive Cancer Center in Rochester, Minnesota, said that while he doesn't have specifics of Chavez's case, "surgery under these circumstances is usually indicated to control a tumor that is growing locally and producing pain."
Having surgery "at this point is likely with the intent of improving the quality of life of the patient and not with a curative intent," Molina said.
Given what Chavez has said about his cancer, it is most likely a soft-tissue sarcoma, said Dr. Michael Pishvaian, an oncologist at Georgetown University's Lombardi Cancer Center in Washington. He said those in the pelvis area have a likelihood of recurring of 50 percent to 70 percent, even with the best treatment.
"I think this is recurrent cancer that at this point is almost certainly not going to go away," Pishvaian said in a telephone interview. "It's unlikely that what he's going through now is curable."
Chavez said he wouldn't have run for re-election this year if tests at the time had shown signs of cancer. He also made his most specific comments yet about his movement carrying on without him if necessary.
"Fortunately, this revolution doesn't depend on one man. We've passed through periods and today we have a collective leadership," Chavez said.
Chavez also recalled a conversation with his mentor Fidel Castro in Havana before his brief trip home to Caracas, in which the Cuban leader referred to a "flame" in Latin America and Chavez's socialist movement.
"A revolution rose up here in Venezuela," Chavez said, adding that "it's been up to us, some of us... to assume responsibilities, assume vanguard roles."
As he concluded his remarks, the president said: "Chavez in truth is no longer only this human being. Chavez is a great collective."
(Copyright 2012 by The Associated Press. All Rights Reserved.)