RIYADH: King Abdullah bin Abdulaziz , the absolute ruler of the world’s top oil exporter and the spiritual home of Islam died on Friday and his half-brother and crown prince Salman replaced him as the new King.

The kingdom’s royal court said in a statement that Abdullah, believed to be around 90, died at 1:00 am local time, expressing its “great sadness and mourning”.

The royal court did not disclose the cause of Abdullah's death, but he was hospitalised in December suffering from pneumonia and had been breathing with the aid of a tube.

The late King Abdullah hailed from Al-Saud dynasty, which gave its name to the Gulf Arab monarchy of Saudi Arabia ruled by the sons of founder Abdul Aziz bin Saud.

Al-Sauds trace their origins to the 1700s, when Saud bin Mohammad reigned as a local sheikh in the central Arabian peninsula, the birthplace two centuries later of the only country in the world named after a family. His son Mohammad allied himself in 1744 with a fiery cleric, Mohammad bin Abdul Wahhab, who advocated a return to purist Islam, spreading the doctrine through the might of the sword.

Mohammad's descendants lost control to Ottoman forces in 1818, but six years later the Saud family retook the desert seat of Riyadh only to squabble over the succession.

By 1902, Abdul Aziz bin Saud expelled the rival Rashidi clan from Riyadh, at the heart of the peninsula, and set about consolidating his power. Abdul Aziz unified the territories in a series of tribal wars.

He took control of the Gulf coast in 1913, expelled Hussein, the Hashemite sharif of Islam's holy city of Mecca, by 1925, and in 1932 declared himself king of Saudi Arabia.

To strengthen his authority, the new king took in marriage the daughters of tribal chiefs. Today, the ruling family totals up to an estimated 25,000 members, including some 200 influential princes.

Oil was discovered in 1938 in the strictly conservative Wahhabi kingdom, transforming it into one of the world's most affluent economies.

Abdul Aziz had 45 recorded sons. King Saud succeeded the kingdom's founder on his death on November 9, 1953.

Faisal took over on November 2, 1964 after his half-brother was deposed on charges of corruption and incompetence. Saud died in exile in 1969.

The architect of Saudi Arabia's modernisation, Faisal was assassinated by a nephew in March 1975. The assassin was said to be mentally disturbed. Khaled, another half-brother, took over and ruled until his death in 1982, when Fahd became king, appointing Abdullah, two years his junior, as crown prince.

Abdullah ran the day-to-day affairs of state after Fahd suffered a stroke in November 1995.

The popular prince acceded to the throne automatically following the death of his half-brother, despite a 1992 law passed by Fahd which opened the succession to Abdul Aziz's grandsons and not just his sons.

How is the new monarch determined?

The late King Abdullah set new rules for determining the line of succession within Saudi Arabia's vast ruling family to avoid unseemly wrangles in the world's leading oil exporter.

The king also established a new mechanism for declaring the reigning monarch or his designated successor unfit to carry out their duties, temporarily or permanently.

Abdullah in October 2006 created the “Council of Allegiance” to decide on the succession, a committee of 35 descendants of the kingdom's founder Abdul Aziz bin Saud.

Under Saudi law, the monarch must be a son or grandson of Abdul Aziz, who died in 1953. All Saudi kings to date have been his sons.

Abdullah also set up a medical commission of five members, three of them doctors, to pronounce on the ability of the king or crown prince to continue in his duties.

Saudis flood Twitter with mournful tweets

A large number of people from around the world swarmed Twitter to express their reaction on the King’s demise.

Abdullah was “loved by the Saudi people and the entire Muslim population.We did not lose a king today, we all lost a father,”Ameera Al Taweel said in one of thousands of Twitter messages.

Tweets by some who disagreed with the King’s approach on human rights and gender equality were less flattering.

Campaigners for women's right to drive referred only in passing to the king's death, saying on their Twitter account: “For all creatures whether big or small -- nothing remains but your deeds and your grave -- and only God lasts forever”.

Key facts on Saudi Arabia

Saudi Arabia is home to the two holiest sites in Islam, Mecca and Medina and is a pilgrimage destination for Muslims from all over the world.

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, the country is approximately four times the size of France.

Population: 28.83 million (2013), including around eight million foreign nationals.

Capital: Riyadh.

Religion: Sunni Islam is the official religion, although about 10 percent of Saudis are Shiites. Other religions are banned from practising in Saudi Arabia, which is home to the Islamic holy cities of Mecca and Medina.

Recent history: Between 2003 and 2006, the kingdom witnessed a wave of Al-Qaeda attacks, including against foreigners.

In recent years there have also been protests and limited confrontations with police in mainly Shiite regions of the east, while women have campaigned for basic rights, such as being allowed to drive.

The country was little affected by the wave of uprisings known as the “Arab Spring” in 2011. King Abdullah announced $36 billion of social benefits at the time, in a clear bid to keep the population content.

In September, Saudi Arabia and other Gulf countries joined the US-led air campaign against the extremist Islamic State group that seized swathes of Iraq and Syria.

Political institutions: An absolute monarchy, with senior princes of the al-Saud family holding the most powerful ministerial positions, all provincial governorships and top military jobs.

The Shura Council makes recommendations on legal and policy issues. There is no constitution and law is based on Islamic sharia in the ultra-conservative Wahhabi-Hanbali tradition. Judges are clerics schooled in sharia.

Economy: At 265.4 billion barrels, Saudi Arabia possesses the second-largest known reserves of crude oil in the world, after Venezuela.

A close ally of the United States, it is a leading member of the Organization of Petroleum Exporting Countries and often acts to regulate world oil prices by adjusting its output.

Saudi Arabia also has significant quantities of natural gas.

The government has pushed to diversify by building petrochemical plants and energy-intensive heavy industry. Pilgrims also fuel an economy which is heavily reliant on foreign workers.

GDP: $745.3 billion in 2013; per capita gross national income: $26,200 in 2013 (World Bank, 2014).

Miltary: 233,500 active duty personnel (IISS, 2012).