* Socialist leader due to start new term on Thursday
* Not seen or heard in public since cancer surgery month ago
* Opposition say government is acting unconstitutionally
* Bolivian, Uruguayan presidents coming anyway
By Diego Ore and Andrew Cawthorne
CARACAS, Jan 8 (Reuters) - Venezuela's opposition demanded on Tuesday to know if President Hugo Chavez was well enough to be sworn in for a new term this week, and two Latin American leaders confirmed they would come even in his absence.
The government says the formal Jan. 10 start of the cancer-stricken Chavez's new six-year term can be postponed with no consequences, while the opposition says a caretaker president should be appointed if Chavez is unable to be sworn in.
The 58-year-old socialist leader, who has dominated the South American OPEC nation since 1999, has not been seen or heard from in public since his Dec. 11 cancer surgery in Cuba.
Official medical bulletins suggest he is too sick to return for Thursday's swearing-in ceremony - though Venezuela's government has not confirmed that.
"I really lament that just 48 hours away the government is incapable of telling Venezuelans if President Chavez is or is not going to be taking office," opposition leader Henrique Capriles, who lost last year's presidential election to Chavez, said in a press conference.
Capriles is expected to run again for the presidency if Chavez dies or is unable to return to office, and a new election is triggered.
"I don't understand why it's so difficult for them to speak the truth," Capriles added, urging the Supreme Court to provide a legal interpretation of the constitution.
Political acrimony over the Jan. 10 swearing-in has taken the focus away from Chavez's physical condition: the government acknowledges he has a serious lung infection after the operation, but denies rumors he is on his death-bed.
The Supreme Court, dominated by Chavez allies, is expected to rule on a vague constitutional article which lays out Jan. 10 as the inauguration date but does not explicitly state what happens if a president is not sworn in at that time.
On the street, Venezuelans are bemused and anxious.
Supporters hold vigils for Chavez's recovery, while opposition activists accuse the president's allies of a Cuban-inspired manipulation of the situation.
Vice-President Nicolas Maduro, who Chavez last month named as his successor, has taken over day-to-day running of government and looks set to continue in the role past Thursday.
The mustachioed former bus driver lacks Chavez's charisma and deep popularity among Venezuelans, but has been seeking to imitate his style with rambunctious attacks on the opposition and televised, ribbon-cutting ceremonies.
"Venezuela is not a monarchy. Ours is not the Cuban system where power is passed around without an election," said Capriles, who argues that National Assembly head Diosdado Cabello should take over on Jan. 10 as mandated by the constitution if there is a formally-declared absence of the president.
The opposition Democratic Unity coalition, whom Capriles ran for in the 2012 vote, has taken its constitutional complaint to the hemispheric Organization of American states - though it has garnered little support in the region.
Rather, Latin American heavyweight Brazil has said it believes Venezuela's government is sticking to the constitution.
The U.S. government, so long the target of Chavez's ire, has said it is up to Venezuelans to decide their future - though it angered Caracas by calling Chavez's policies "authoritarian."
At least two fellow leftist presidents - Evo Morales of Bolivia and Jose Mujica of Uruguay - are coming to Caracas for Thursday's ceremony, where it appears there will be a rally in solidarity with the ailing Chavez in place of the inauguration.
Argentina's left-leaning President Cristina Hernandez has announced plans to visit Chavez in Havana on Friday, a move that would appear to confirm that Chavez will miss the swearing-in even though Cabello said there was a chance he could attend.
Underlining the gravity of his condition, Mujica's wife said the Uruguayan leader had been dissuaded from visiting Cuba.
"He first checked if he could go to Havana. But he (Chavez) cannot be seen. So I think going to Havana would be a bother," Mujica's wife, Lucia Topolansky, said in a radio interview.
"With all respect, I urge our Latin American presidents not to play the game of one political party," Capriles added at his news conference, saying the ruling Socialist Party was trying to cook up Venezuela's future over the heads of the people.
With the micro-managing and authoritarian 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 in-fighting within the Socialist Party have not materialized, with Maduro and Cabello in particular pledging unity despite rumors of rivalry.
Having failed to topple Chavez a decade ago with sustained street protests and national strikes, the opposition's tactics are to show respect for his health problems while criticizing the legalities of the situation and preparing for a possible new presidential election later in 2013.
Venezuelan bond prices, which had soared in recent weeks on Chavez's health woes, dipped on Monday and Tuesday as investors' expectations of a quick government change apparently dimmed.
"The 'regime change' euphoria seems excessive taking into account the unclear legal transition and perhaps, more importantly, the risk that regime change does not allow for policy change," New York-based Jefferies' managing director Siobhan Morden said in a note on the bonds.