* Chavez still in Cuba recovering from cancer surgery
* Due to take office Jan. 10 after reelection for third term
* Opposition says Chavez too sick to govern, eyes new vote
By Brian Ellsworth and Eyanir Chinea
CARACAS, Dec 22 (Reuters) - Venezuela will not call fresh elections if Hugo Chavez's cancer prevents him from taking office by Jan. 10, the head of Congress said on Saturday, despite a constitutional mandate that the swearing-in take place on that date.
Chavez is recovering in Cuba from a six-hour cancer operation that followed his October re-election. The socialist leader has not been heard from for nearly two weeks, raising doubts as to whether he will be fit to continue governing.
Opposition leaders may pounce on the issue of the swearing-in date to demand that authorities call fresh elections because of Chavez's apparently critical state of health due to an undisclosed type of cancer in the pelvic region.
A constitutional dispute over succession could lead to a messy transition toward a post-Chavez era in the South American nation with the world's largest oil reserves.
"Since Chavez might not be here in on Jan. 10, (the opposition) hopes the National Assembly will call elections within 30 days. They're wrong. Dead wrong," said Diosdado Cabello, the National Assembly's president and one of Chavez's closest allies, during a ceremony to swear in a recently elected governor.
"That's not going to happen because our president is named Hugo Chavez, he was reelected and is in the hearts of all Venezuelans."
He suggested Chavez may need more time to recover from his surgery. Officials in recent weeks have recognized his condition was serious, and the garrulous leader's unusual silence has built up alarm even among supporters.
The constitution says "the elected candidate will assume the Presidency of the Republic on January 10th of the first year of their constitutional term, via swearing-in by the National Assembly."
It says new elections are to be called if the National Assembly determines a "complete absence" of the president because of death, physical or mental impairment or abandoning the job.
The opposition believes it would have a better shot against Chavez's anointed successor, Vice President Nicolas Maduro, than against the charismatic former soldier who for 14 years has been nearly invincible at the ballot box.
Chavez allies want to avoid a public debate over the president's health because his cancer has been treated as a state secret. His treatment in communist Cuba has helped keep his condition under wraps, and the Venezuelan government has given only terse and cryptic statements about his post-operation recovery.
Constitutional lawyer Jose Vice Harold said he expects the Supreme Court, which is controlled by Chavez allies, will rule that Chavez may extend his existing term without having to be sworn in with the expectation that he will eventually recover.
"What they are doing is taking the debate over succession from the National Assembly, which is where it belongs, and moving it to the Supreme Court where behind closed doors they can decide the next steps are," said Harold, a Chavez critic and constitutional law professor as the Universidad Catholic Andres Bellow.
Chavez has vastly expanded presidential powers and built a near-cult following among millions of poor Venezuelans, who love his feisty language and oil-financed social welfare projects.
Opposition leaders are smarting from this month's governors elections in which Chavez allies won 20 of 23 states. They are trying to keep attention focused on day-to-day problems from rampant crime to power outages.