After Bongo's prime minister confirmed news of the 73-year-old's death in a private clinic in Barcelona, Spain, officials in the west African nation announced 30 days of national mourning, with all flags flown at half-mast.

Bongo's body is to be repatriated to Libreville on Friday, so a ceremony can be held in Spain, a Gabon official told AFP in the Spanish port city. The official funeral will be held in Gabon between Friday and June 15, he added.

Bongo's 50-year-old son Ali Ben Bongo, Gabon's defence minister and favourite to succeed him, renewed appeals for public calm -- indicating his pivotal role in coming days and weeks.

"I am calling for calm and serenity of heart and reverence to preserve the unity and peace so dear to our late father," he said in a televised appeal after his ministry announced the closure of air, land and sea borders.

Prime Minister Jean Eyeghe Ndong announced in a statement earlier: "At 2:30 pm, the medical team informed me, as well as the officials and members of the family present, that the president of the republic, head of state Omar Bongo Ondimba had just passed away following a heart attack."

Bongo, in power since 1967, was understood to have been treated for intestinal cancer. Ndong called on the Gabonese people to "remain united and stand together in contemplation and dignity."

The African Union led continental tributes, which also came in from the Central African Republic, Congo-Brazzaville, Ivory Coast, Morocco's King Mohammed VI, Senegal and UFR rebels in Chad.

Interviewed by AFP at AU headquarters in Addis Ababa, peace and security commissioner Ramtane Lamamra said a "very important son of Africa" had died.

Lamamra expressed his hope that Bongo's death "will not result in any difficulties for Gabon in its implementation of its constitution during the transition of power."

US President Barack Obama said Bongo "played a key role in developing and shaping the strong bilateral relationship that exists between Gabon and the United States today.

"His work in conservation in his country and his commitment to conflict resolution across the continent are an important part of his legacy and will be remembered with respect."

Bongo came to power in 1967 with French support and ruled over a state that grew rich on abundant oil while most of the 1.5 million population remained poor.

His demise raised fears of a power vacuum and initial reports of his death on Sunday night prompted Libreville residents to stock up on fuel.

On Monday, police and troops were posted at key points across the capital, while residents complained that Internet access to international news had been cut in various parts of the country and telephone lines severely disrupted since Sunday.

Gabon Telecom operator said the interruption was accidental.

"The incident shut down more than half of international phone lines, data transmissions and the Internet," it said in a statement.

Libreville shut up shop, with bars and night clubs closing until further notice. All rallies were banned. Security worker Ismael Joseph Essono said: "In Africa, when a leader dies, it's desolation. Think about it: 41 years in power, the people are in mourning."

Bongo's final months were marked by a row with Paris over a French inquiry into luxury properties he had bought in France and claims by anti-corruption activists they were acquired with embezzled state funds.

A French court decision in February to freeze Bongo's bank accounts added fuel to the fire, and his government accused France of waging a "campaign to destabilise" the country.

Plaintiffs' lawyer William Bourdon said Monday that Bongo's death "changes nothing" in terms of legal proceedings, underlining that "several members of his family" remain in his clients' sights.

Gabon was the first African country to host French oil giant Elf in the 1960s, from where the company operated as a state within a state, serving as a base for French military and espionage activities.

The Paris trial in 2003 of former Elf chairman Loik Le Floch-Prigent revealed the extent of shady dealings.

"Gabon without France is like a car with no driver. France without Gabon is like a car with no fuel," is how Bongo described the relationship between Paris and its former colony, home to some 12,000 French expats.