CARACAS — President Hugo Chavez headed back to Cuba on Sunday for a third cancer surgery after naming his vice president as his choice to lead Venezuela if the illness cuts short his presidency.
Chavez’s announcement on Saturday night cast a shadow of uncertainty over the country, and thousands of his supporters poured into city plazas across the nation to pray for his recovery from what appears to be an aggressive type of cancer.
Chavez acknowledged the seriousness of his health problem in a televised address. He said 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.
Several outside medical analysts said that based on Chavez’s account of his condition and treatment so far, they doubt the cancer can be cured.
Chavez said he has not given up.
‘‘With the grace of God, we’ll come out victorious,’’ he said, holding up a crucifix and kissing it during his Saturday night appearance.
The 58-year-old president is scheduled to be sworn in for a new six-year term Jan. 10. He has been in office for nearly 14 years, since 1999.
‘‘There are risks. Who can deny it?’’ Chavez said, seated at the presidential palace beside Maduro and other aides.
Chavez, who won reelection Oct. 7, said he would undergo surgery in Havana in the coming days. Lawmakers on Sunday unanimously agreed to grant him permission to leave the country for the operation.
The National Assembly also said Maduro should take on his duties during his temporary absence, as the constitution specifies. Opposition lawmaker Julio Borges criticized the incomplete information released on Chavez, saying: ‘‘Venezuela has a right to know the truth.’’
Chavez has kept secret details about his illness, including the precise location of tumors and the type of cancer. He has said he travels to Cuba for treatment because his cancer was diagnosed by doctors there.
National Assembly President Diosdado Cabello said there are no plans at this time for Chavez to cede power, even temporarily, as president.
Under the Venezuelan Constitution, the vice president 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.
The president, who had just returned from Cuba early Friday, said on television late Saturday that tests had found a return of ‘‘some malignant cells’’ in the same area where tumors were previously removed.
Chavez made his most recent trip to Cuba on Nov. 27. His 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 behind him.