* Chavez not heard from since Dec. 11 cancer surgery
* Opposition accuses government of violating constitution
Venezuela will postpone the inauguration of President Hugo Chavez for a new term due to health problems, the government said on Tuesday, another sign the socialist leader's cancer may be bringing an end to his 14 years in power.
The 58-year-old former soldier who has dominated the South American OPEC nation since 1999 has not been heard from since surgery on Dec. 11 in Cuba - his fourth operation since he was diagnosed with an undisclosed type of cancer in June 2011.
The delay has outraged opposition leaders who insist that Chavez must be sworn in before the National Assembly on Jan. 10 as laid out in the constitution, or temporarily step aside and leave an ally in power.
"The commander president wants us to inform that, based on his medical team's recommendations, the post-operative recovery should extend past Jan. 10," Vice President Nicolas Maduro said in a letter read to the legislature.
"As a result, he will not be able to be present at the National Assembly on that date."
The letter said authorities would seek another date for the inauguration ceremony but did not say when it would take place, nor give any time frame for Chavez's recovery or his return from Havana.
Rather than being sworn in by the legislature, he would take his oath at a later date before the Supreme Court, the letter said, as allowed by the constitution.
Government leaders insist Chavez is completely fulfilling his duties as head of state - even though official medical bulletins say he has a severe pulmonary infection and has had trouble breathing.
The government has called for a massive rally of supporters outside the presidential palace on Thursday, and allies including Uruguayan President Jose Mujica and Bolivian leader Evo Morales have confirmed they will visit Venezuela this week despite Chavez's absence.
But the unprecedented silence by the president - famous for regularly speaking for hours in meandering broadcasts - has left many convinced he could be in his last days.
His resignation or death would upend politics in the oil-rich nation where he enjoys a deity-like status among poor supporters thankful for his social largesse.
His critics call him a fledgling dictator who has squandered billions of dollars from crude sales while dashing the independence of state institutions.
The constitution does not specify what happens if the president does not take office on Jan. 10.
Opposition leaders argue that Congress chief Diosdado Cabello should take over as mandated by the constitution if the president's absence is formally declared. Cabello, a close Chavez ally, has ruled that out, saying the president continues to be in charge.
"Venezuela is not a monarchy. Ours is not the Cuban system where power is passed around without an election," opposition leader Henrique Capriles, who lost to Chavez in the October presidential election, told reporters on Tuesday.
Supporters have held near-daily vigils for Chavez's recovery, while opposition activists accuse the president's allies of a Cuban-inspired manipulation of the situation.
Vice President Maduro, who Chavez named last month as his successor, has taken over the day-to-day running of the government and looks set to continue in the role past Thursday.
The mustachioed former bus driver lacks Chavez's charisma, but he has sought to imitate the president's style with rambunctious attacks on the opposition and televised ribbon-cutting ceremonies.
With the micro-managing Chavez away, major policy decisions in Venezuela, such as a widely expected devaluation of the bolivar currency, appear to be on hold.
Opposition predictions of fighting within the ruling Socialist Party have not materialized, however, with Maduro and Cabello in particular pledging unity despite rumors of rivalry.