CARACAS (AFP) - Venezuelan President Hugo Chavez is in "stable" but still delicate condition, more than three weeks after undergoing a difficult cancer surgery in Cuba, his son-in-law said.

"The medical team explains to us that President Chavez's condition continues to be stable within a delicate framework," Mr Jorge Arreaza, who is also Venezuela's minister of science and technology, said on Twitter on Wednesday.

His comment came after Venezuela's main opposition movement called on the government to tell "the truth" about the president's health as one of his closest allies said his condition was "very worrying."

"Let's hope our prayers will be effective in saving the life of brother President Chavez," Bolivian President Evo Morales said of Mr Chavez.

More than three weeks after undergoing his fourth round of cancer surgery in Havana, the government faced mounting pressures to lay out Mr Chavez's health in detail and put in place an orderly succession process should he die or be incapacitated.

The head of an opposition umbrella group - the MUD - accused the government of "outlandish irresponsibility" in trying to make it appear that Mr Chavez was exercising his duties as president as he underwent a difficult recovery in Cuba.

"It is essential that the government act in a manner that gives confidence. It is essential that it tell the truth," said Mr Ramon Guillermo Aveledo. He demanded that the government provide a "a diagnosis and a medical prognosis" on the 58-year-old's condition.

The government has provided only sketchy information about Mr Chavez's battle with cancer, withholding even the type of cancer and the prognosis since the disease was first detected by Cuban doctors in June 2010.

Information about his progress has come in vague, often upbeat comments by close aides and close allies like Bolivia's president.

Mr Morales told reporters in Bolivia on Wednesday that he had spoken with the Venezuelan leader's family, and that "our brother President Chavez's situation is very worrying."

"It is very painful for me," the Bolivian leader acknowledged.

Vice-President Nicolas Maduro said in a television interview from Havana on Tuesday that Mr Chavez was conscious and fully aware "of how complex the post-operative condition is."

"At times there have been slight improvements, at times there have been stationary situations."

But he provided few specifics about the president's health even as he accused the Venezuelan right of deliberately spreading what he said were sick lies and rumours about Mr Chavez's condition.

A cascade of rumours have swept social networks and Internet blogs, with some claiming that the president was dead and others that he was on life support.

Before leaving for Cuba, Mr Chavez had named Mr Maduro as his choice to succeed him but after having dominated national life here for 14 years, his departure from the scene was expected to leave an unsettling power vacuum.

The Venezuelan constitution calls for new elections within 30 days if the president-elect is unable to take the oath of office.

It was also unclear whether Mr Maduro, who was appointed vice-president by Mr Chavez after his re-election on Oct 7, could remain in office after Jan 10.

"On Jan 10, a new constitutional term begins. If the president shows up, he shows up. If the president does not show up, the president of the National Assembly is in line to assume the presidency temporarily, according to the Constitution," said Mr Aveledo.