* Chavez silent for three weeks after cancer surgery
* Opposition seeks more details on his condition
* Chavez illness could delay swearing-in for new term
By Diego Ore and Brian Ellsworth
CARACAS, Jan 2 (Reuters) - Venezuela's opposition on Wednesday demanded the government tell "the whole truth" about the health of cancer-stricken President Hugo Chavez, who has not been heard from in three weeks after undergoing a grueling operation in Cuba.
Officials have acknowledged the usually garrulous former soldier's health is delicate after his fourth cancer surgery in 18 months, but they have offered scant details on his condition.
He has not spoken in public in more than three weeks.
Ramon Aveledo, head of the opposition Democratic Unity coalition, slammed the government for not keeping its word about keeping Venezuelans informed.
"The official version (of Chavez's health) hides more information than it gives," Aveledo said at a press conference.
"The vice president himself has promised to tell the truth, whatever it is. Fine, he should tell it. He should tell the whole truth," said Aveledo.
Vice President Nicolas Maduro, whom Chavez last month designated as his heir apparent, on Tuesday said in an interview from Havana that Chavez had recognized the complexity of his post-operative condition.
Maduro said he was returning to Venezuela after several days visiting with Chavez and his relatives, which may quell rumors his trip to Cuba signaled the president was in his final days.
The president's son-in-law and Science Minister Jorge Arreaza, who is in Havana, said via his Twitter account on Wednesday that the medical team told him Chavez's condition "remains stable" but that his health is still delicate.
"Commander Chavez is fighting hard and he sends his love to the people. Dedication and patience!!!" he tweeted.
Chavez's abrupt exit from the political scene would be a shock for Venezuela, where his oil-financed socialism has made him a hero to the poor majority but a nemesis to critics who call him a dictator.
He is still set to be sworn in on Jan. 10, as laid out in the constitution. If he dies or steps aside, new elections would be held within 30 days, with Maduro running as the Socialist Party candidate.
Chavez suffered unexpected bleeding and a respiratory infection after a six-hour operation on Dec. 11. Terse official statements have said nothing about when he might be expected back or whether his life is in danger.
The government has provided none of the signature videos or pictures released after Chavez was diagnosed with cancer in June 2011 and his relapse in 2012. And allies have refused to discuss the possibility that he could hand over power or resign.
Chavez last year staged what appeared to be remarkable comeback from the disease to win reelection to a third six-year term in October despite being weakened by radiation therapy. He returned to Cuba for new treatment within weeks of his win.
Officials from the ruling Socialist Party are now suggesting his inauguration could be postponed indefinitely to accommodate his health.
Aveledo insisted the government should stick to the Jan. 10 timeline called for in the constitution.
"Trying to make the country believe that the president is governing is absurd to the point of being irresponsible," he said. "Jan. 10 marks the end of one presidential term and the start of another. As such, there is no continuation of the current government."
Aveledo said if Chavez cannot make it back in time, he should hand power over to the president of Congress - who would temporarily run the country while elections are called.
Congress, controlled by Chavez allies, on Saturday elects a new president. Current Congress chief Diosdado Cabello, a close Chavez ally who could be reelected to head the legislature, has at times been considered a rival of Maduro. The two have taken great pains in recent weeks to publicly deny this.
While the constitution cites Jan. 10 as the start of the new term, it does not explicitly state what happens if the president does not take office on that date.
Chavez's condition is being watched closely by Latin American countries that have benefited from his generous assistance, as well as Wall Street investors who are drawn to Venezuela's lucrative and heavily traded bonds.