* Chavez not heard from since cancer surgery month ago
* Opposition say government is acting unconstitutionally
CARACAS, Jan 8 (Reuters) - Venezuela will postpone Thursday's presidential inauguration due to President Hugo Chavez's continuing health problems, the government said on Tuesday.
The 58-year-old socialist leader, who has dominated the South American OPEC nation since 1999, has not been heard from or seen in public since his Dec. 11 cancer surgery in Cuba.
The constitution says the president should begin a new term on Jan. 10.
"(Chavez's) medical team has recommended that the postoperative recovery should extend past Jan. 10," Vice President Nicolas Maduro said in a letter read out to the National Assembly.
The letter did not say when the inauguration would take place or provide any time frame for Chavez's recovery.
The delay is another sign that Chavez's battle with an undisclosed form of cancer in the pelvic region may keep him from ruling for a third term. His resignation or death would upend politics in the oil-rich country that has grown accustomed to his charismatic but controversial leadership.
The constitution does not specify what happens if the president does not take office on Jan. 10.
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 troubled breathing.
Opposition leaders insist the government is running roughshod over the constitution by ignoring the specified inauguration date. They insist Congress head Diosdado Cabello, a key Chavez ally, should step in as a temporary president while Chavez recovers.