Bongo, who has led the former French colony in west-central Africa since 1967, had officially been in a clinic in Barcelona for medical check-up but several sources said he was being treated for cancer.
Bongo first entered the Gabonese government in 1965 and became vice president in 1967. He built a powerful dynasty-rule in the country which has benefited from the discovery of oil, although much of the wealth is concentrated in a small proportion of the 1.5 million population. The president's wife, Edith Lucie Bongo Odimba, daughter of Congo President Nguesso, died in Morocco in March at the age of 45, after a long illness.
Opposition parties were allowed in 1990 in Gabon amid a wave of pro-democracy protests. Parliament -- dominated by Bongo's supporters -- removed presidential term limits from the constitution in 2003.
Bongo became the longest-ruling head of state, not counting the monarchs of Britain and Thailand, after Fidel Castro stepped aside in Cuba last year.
While most Gabonese genuinely feared Bongo and he faced little opposition, many accepted his rule because he had kept his country remarkably peaceful and governed without the sustained brutality characteristic of many dictators.
Bongo, meanwhile, amassed a fortune that made him one of the world's richest men, according to Freedom House, a private Washington-based democracy watchdog organisation. No one really knew how much he was worth.
He assumed the presidency on December 2, 1967, after the death of Leon M'Ba, the country's only other head of state since independence from France in 1960. Bongo, then vice president, succeeded M'Ba and set up a one-party state.
Six years later, Bongo converted to Islam and took the name Omar.
Image: Gabon President Omar Bongo in Dakar March 2008.
Photograph: Normand Blouin/Reuters