* El Pais pulls print edition, blames local agency
* Venezuelan president has not been seen in six weeks (Recasts, updates throughout with quotes, details)
By Fiona Ortiz and Daniel Wallis
MADRID/CARACAS, Jan 24 (Reuters) - Spain's influential El Pais newspaper apologised on Thursday for splashing a "false photo" of Venezuela's cancer-stricken leader Hugo Chavez on its front page, prompting a furious response from the government in Caracas, which vowed to take legal action.
Within minutes of posting the image online as a global exclusive, El Pais said it had discovered from social media that the photo was not of Chavez. It removed it from its website and withdrew its print edition.
Venezuela's government said the publication of the photo - which showed the head of a man lying down with a breathing tube in his mouth - was "grotesque," while Argentinian President Cristina Fernandez, a close ally of Chavez, called it vile.
"El Pais apologises to its readers for the damage caused. The newspaper has opened an investigation to determine the circumstances of what happened and the errors that were committed in the verification of the photo," the paper said.
Chavez, 58, is fighting to recover in Cuba after undergoing his fourth cancer operation in just 18 months. He has not spoken or appeared in public for six weeks, fuelling speculation about how serious his condition is.
El Pais, one of the world's biggest Spanish-language publications and an institution both in Spain and in Latin America, said it received the grainy image from the agency Gtres Online, which it said represents 60 other agencies in Spain.
In a statement, El Pais said the newspaper was told it had been taken seven days earlier by a Cuban nurse who was part of Chavez's medical team, and was then sent to the nurse's sister, who lives in Spain.
"The agency has acknowledged it was deceived by those who provided the material and will take legal action," El Pais said.
The photo was on the newspaper's website for half an hour and also appeared in early editions of the print version that were then pulled from newsstands and replaced with a new edition with a different front page.
In Venezuela, anxious Chavez supporters and opponents alike are waiting for any new picture, video or audio message from the socialist leader, who is famed for filling the airwaves with long-winded speeches, jokes and withering jabs at his foes.
NO SIGHT OF CHAVEZ
Officials say his condition is improving after he suffered multiple complications, including unexpected bleeding and a severe respiratory problem following the Dec. 11 surgery.
But, in contrast to Chavez's previous visits to Havana, officials have not published any evidence of his condition. In 2011, with great fanfare, they broadcast video footage of him reading a newspaper, walking in a garden, and chatting with his friend and mentor, Cuba's ex-leader Fidel Castro.
In the absence of such proof this time, many Venezuelans are questioning the terse official bulletins and suspect Chavez's extraordinary 14 years in power could be coming to an end.
The president has never said exactly what type of cancer he has, only that the initial tumour found in mid-2011 was in his pelvic area and was the size of a baseball.
Venezuelan opposition leaders have long accused the government of secrecy over his illness, while supporters accuse "bourgeois" local and foreign media of being in league with the opposition to spread rumours he is at death's door.
The handling of information relating to Chavez's health has become as contentious as the man himself, and his administration's updates have been confusing and contradictory.
The government says it has never been more transparent. It described El Pais's publication of the picture - a screengrab from an unrelated 2008 video - as part of efforts by far-right political forces to attack Chavez's self-styled revolution.
It said it would take appropriate legal action, and that the newspaper's apology to its readers was not enough.
"Neither their disgusting photos nor their systematic campaigns will stop the president's advance," Information Minister Ernesto Villegas told a news conference in Caracas.
"Would El Pais publish a similar photo of a European leader? Of its director? Sensationalism is valid if the victim is a revolutionary 'sudaca'," he added, using a pejorative term that is sometimes used in Spain to refer to Latin Americans. (Editing by Eric Walsh)