Sultan Qaboos bin Said Al Said of Oman, the Arab world's longest-serving ruler, has died aged 79.
"With great sorrow and deep sadness... the royal court mourns His Majesty Sultan Qaboos bin Said, who passed away on Friday," a court statement said.
Last month he returned home after undergoing medical checks and treatment in Belgium. There were reports he was suffering from cancer.
Sultan Qaboos was unmarried and had no heir or designated successor.
Three days of national mourning have been declared.
The sultan 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.
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 Sultan Qaboos secretly recorded his choice and enthrone that person.
The leading contenders reportedly include three brothers who are cousins of the late sultan: 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 was a royal advisor.
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.