LIBREVILLE, Gabon - Gabon President Omar Bongo, 73, whose 42-year rule, the longest of any president in the world, was a throwback to an era when Africa was ruled by "Big Men," died of cardiac arrest in a Spanish hospital yesterday.

Doctors at the Quiron Clinic in Barcelona announced the death, Gabonese Prime Minister Jean Eyeghe Ndong said. The president was admitted to the hospital last month.

In Gabon, the government responded by announcing the closure of the West African country's international airport and all of its land and sea borders. The mood in Libreville, the capital, appeared tense, as security forces fanned out and took positions in front of key government buildings and important electrical installations.

People rushing to get home as soon as the news was announced caused huge traffic jams. Some residents could be seen hurrying home with rice bags, apparently to stockpile food in advance of possible store closures.

The country's constitution calls for the Senate leader to assume power and organize presidential elections within 90 days of the president's death. But there has been speculation that one of his sons - who is minister of defense - would try to seize power upon his father's death, as happened in nearby Togo.

President Bongo, who was believed to be one of the world's wealthiest leaders, became the longest-ruling head of government - a category that does not include the monarchs of Britain and Thailand - when Cuba's Fidel Castro handed power to his brother last year.

He had kept a tight grip on power in the oil-rich former French colony since he became president in 1967, and his ruling party has dominated the country's parliament for decades. Opposition parties were not allowed until 1990, amid a wave of pro-democracy protests.

Since then, elections in the nation of 1.5 million people have been marred by allegations of rigging and unrest. In 2003, parliament - dominated by his supporters - removed presidential term limits from the constitution.

Born Albert Bernard Bongo on Dec. 30, 1935, the youngest of 12 children, he served as a lieutenant in the French air force, then climbed quickly through the civil service, eventually becoming vice president. He assumed the presidency Dec. 2, 1967, after the death of Leon M'Ba, the country's only other head of state since independence from France in 1960.

President Bongo set up a one-party state. Six years later, he converted to Islam and took the name Omar.

While most Gabonese genuinely feared him and there was little opposition, many accepted his rule because he had kept his country remarkably peaceful and governed without the sustained brutality of many dictators.