Sultan Qaboos bin Said Al Said of Oman, the Arab world's longest-serving ruler, has died aged 79, state media say.
Last month he returned home after undergoing medical checks and treatment in Belgium. There were reports he was suffering from cancer.
Sultan Qaboos deposed his father in a bloodless coup with British support in 1970. Using its oil wealth, he then set Oman on a path to development.
He was unmarried and had no heir or designated successor. Sultan Qaboos died on Friday evening, state media reported early on Saturday. Three days of national mourning have been declared.
According to the sultanate's Basic Statute, the Royal Family Council - comprising about 50 male members - should choose a new sultan within three days of the throne falling vacant.
If the family cannot agree, members of the defence council and the chairmen of the Supreme Court, the Consultative Council and the State Council will open a sealed envelope in which Qaboos secretly recorded his choice and enthrone that person.
The leading contenders reportedly include three brothers who are cousins of Qaboos: Culture Minister Haitham bin Tariq Al Said; Deputy Prime Minister Asaad bin Tariq Al Said; and Shihab bin Tariq Al Said, a former Oman Navy commander who advises the sultan.
The sultan is the paramount decision-maker in Oman and also holds the positions of prime minister, supreme commander of the armed forces, minister of defence, minister of finance and minister of foreign affairs.
For almost five decades, Sultan Qaboos completely dominated the political life of Oman, which is home to 4.6 million people, of whom about 43% are expatriates.
At the age of 29 he overthrew his father, Said bin Taimur, a reclusive and ultra-conservative ruler who banned a range of things, including listening to the radio or wearing sunglasses, and decided who could get married, be educated or leave the country.