* Africa's longest serving leader in Europe for treatment
* Opposition worries president's son ready to succeed
LIBREVILLE, May 16 (Reuters) - Gabonese President Omar Bongo, Africa's longest-serving ruler, has travelled to Europe and temporarily stood down from his duties as head of state on health grounds, sources said on Saturday.
The absence of 73-year-old Bongo, who has ruled the oil-producing West African country since 1967, risks creating a power vacuum into which opposition leaders fear the president's son Ali Ben Bongo, currently defence minister, will step.
"The president's son ... would be ready to succeed," Pierre Mamboundou, leader of the main opposition party, the Union of Gabonese People, said on state radio.
Bongo has faced mounting speculation about the state of his health and has been in hospital for treatment at least twice in recent months.
"The president is taking care of himself and resting," said a senior presidential adviser, speaking on condition of anonymity. "The president has taken three weeks of rest after having had more than three months of intense activity," he said.
Bongo flew to Europe in a medically equipped aircraft last week, medical sources close to the capital's airport said.
Sources in Libreville suggest Bongo is in Spain, though neither Gabon nor Spain has confirmed this.
Last Thursday, Bongo suspended his functions as head of state for the first time in his 42-year rule to mourn his late wife, who died in March after a long illness. [ID:nL71001808]
At the time, the government did not specify how long the suspension would last but said that if needed, Senate president Rose Francine Rogombe, a 66-year-old lawyer in the ruling Gabonese Democratic Party (PDG), would step in as caretaker.
Bongo is one of three African presidents with close ties to France who have been targeted by a French judicial inquiry into their assets, initiated after a complaint filed by an anti-corruption group.
Citing a 2007 police inquiry that found that the ruling families in Gabon, Congo Republic and Equatorial Guinea had acquired cars and luxury homes in France worth tens of millions of euros, Transparency International alleged these assets were bought with embezzled public money.
Writing by Daniel Magnowski; Editing by Mark Trevelyan
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