Saudi Arabia's King Abdullah died early on Friday and his brother Salman became king, the royal court in the world's top oil exporter and birthplace of Islam said in an official statement.
King Salman has named his half-brother Muqrin as his crown prince and heir.
"His Highness Salman bin Abdulaziz Al Saud and all members of the family and the nation mourn the Custodian of the Two Holy Mosques King Abdullah bin Abdulaziz, who passed away at exactly 1 a.m. this morning," said the statement carried by state television.
Abdullah, said by the Saudi embassy in Washington to have been born in 1924, had ruled Saudi Arabia as king since 2005, but had run the country as de facto regent for a decade before that after his predecessor King Fahd suffered a debilitating stroke.
President Barack Obama expressed condolences and saluted the late king's commitment to close U.S.-Saudi ties.
"As a leader, he was always candid and had the courage of his convictions," Obama said in a statement. "One of those convictions was his steadfast and passionate belief in the importance of the U.S.-Saudi relationship as a force for stability and security in the Middle East and beyond."
President George H. W. Bush who sent an American army to Saudi Arabia to help repel Iraqi forces from Kuwait in 1990-91, described Abdullah as "a wise and reliable ally, helping our nations build on a strategic relationship" in a statement.
King Salman will continue Abdullah’s reforms
June 21, 2007 shows French President Nicolas Sarkozy shaking hands with Saudi King Abdullah bin Abdul Aziz al-Saud. (AFP)
King Salman, thought to be 79, has been part of the ruling clique of princes for decades and is thought likely to continue the main thrusts of Saudi strategic policy, including maintaining the alliance with the United States and working towards energy market stability.
During his five decades as Riyadh governor he was reputedly adept at managing the delicate balance of clerical, tribal and princely interests that determine Saudi policy, while maintaining good relations with the West.
"I think he will continue with Abdullah's reforms. He realises the importance of this. He's not conservative in person, but he values the opinion of the conservative constituency of the country," said Jamal Khashoggi, head of a news channel owned by a Saudi prince.
May 14, 2012 shows Saudi Arabian King Abdullah bin Abdul Aziz drinking Arabic coffee during a welcoming ceremony for Gulf Cooperation Council. (AFP)
"King Abdullah was willing to challenge the conservatives, but not to crush them. Salman respected the status quo. He wanted reform but was very much connected to the tribal mentality, the conservative nature of his constituency," he added.
Crown Prince Muqrin was placed firmly in the line of succession by Abdullah a year ago after serving as the kingdom's intelligence chief and later an adviser to the king on foreign and security policy.
He promised to continue Abdullah's reforms, but has also demonstrated a populist edge by demanding banks serve the interests of Saudi citizens more fairly.
October 30, 2007, King Abdullah of Saudi Arabia (Centre Foreground) reviews a Guard of Honour with Britain's Prince Philip (Centre Background) in Horse Guards Parade in London, during a State visit. (AFP)
In the long term Saudi rulers have to manage the needs of a rapidly growing population plagued by structural unemployment, and an economy that remains overly dependent on oil revenue and undermined by lavish subsidies.
A Saudi businessman in Jeddah told Reuters: "People are very sad because they loved him very much. He was a father figure, sincere, and truly a king. He was always trying to be the arbitrator. He kept his word and was known for his loyalty."
Saudi Arabia, whose ruler King Abdullah died on Friday, is an oil-rich, absolute monarchy that occupies most of the arid Arabian peninsula.
It is also home to the two most holy sites in Islam and is a pilgrimage destination for Muslims from all over the world.
Key facts on Saudi Arabia
Geography: Saudi Arabia dominates the Arabian Peninsula, lying between the Gulf and the Red Sea. Mostly desert, it is bordered to the south by Yemen, the east by Oman and the Gulf emirate states, and to the north by Jordan and Iraq.
Area: At 2,149,690 square kilometres (about 860,000 square miles), the country is approximately four times the size of France.
Population: 28.83 million (2013), including around eight million foreign nationals.
History: Bedouin prince Muhammad bin Saud allied himself in 1744 with cleric Muhammad bin Abdul Wahhab to forge a new state. By 1802 they controlled the lands that mostly make up Saudi Arabia, but lost much of it to the Ottomans in 1818. Abdul Aziz bin Saud captured Riyadh in 1902 and declared himself king in 1932. Oil was discovered in 1938. Abdul Aziz died in 1953, and a succession of his sons have since ruled. King Abdullah bin Abdul Aziz inherited the throne in 2005.
Economy: At 265.4 billion barrels, Saudi Arabia possesses the second-largest known reserves of crude oil in the world, after Venezuela. Saudi Arabia also has significant quantities of natural gas.
GDP: $745.3 billion in 2013; per capita gross national income: $26,200 in 2013 (World Bank, 2014).
Military: 233,500 active duty personnel (IISS, 2012).