Nayef, interior minister since 1975 and thought to be 78, was the heir to Saudi King Abdullah and was appointed crown prince in October after the death of his elder brother and predecessor in the role, Crown Prince Sultan.  

State television said the burial would be in Mecca on Sunday.  

His death means the 89-year-old King Abdullah must nominate a new heir for the second time in nine months.  

Defence Minister Prince Salman, 76, seen as likely to continue King Abdullah’s cautious reforms, has long been viewed as the next most senior prince in the kingdom’s succession.  

Nayef had a reputation as a steely conservative who opposed King Abdullah’s reforms and developed a formidable security infrastructure that crushed al Qaeda but also locked up some political activists.  

He, King Abdullah and Salman are among the nearly 40 sons of Saudi Arabia’s founder, Abdulaziz ibn Saud, who established the kingdom in 1935.  

Salman was made defence minister in November and had served as Riyadh governor for five decades.  

Nayef went to Switzerland for medical tests in May.   

He was seen as more conservative than his half brother, and a pragmatist who liked to describe himself as a soldier under the command of the Saudi monarch.

The crown prince was known for his solid relations with the kingdom's religious elite and was believed to have opposed reforms that could liberalise the Gulf's ultra-conservative Islamic society.

He was known for his suspicion and mistrust of Saudi Arabia's arch-rival Iran, and had pushed for hardline policies towards the Shi’ite nation.

Interior minister for more than three decades, Nayef enjoyed strong relations across the Arab region.

Diplomats said he played a key role in Riyadh's decisions to host Tunisia's ousted strongman Zine El Abidine Ben Ali after his January 2011 overthrow, and to send troops in March last year to Bahrain to help end Shiite-led protests.

Born in the western city of Taif in 1933, Nayef was quickly pushed into public service, being named governor of Riyadh when he was barely 20.

His elder brother Fahd brought him into the interior ministry, where he was named deputy minister in 1970 and minister five years later, when Fahd became crown prince.

Nayef was credited for the successful crackdown on al Qaeda militants in subsequent years, halting their wave of bloody attacks on the kingdom between 2003 and 2006.

His internal security campaign forced al Qaeda leaders and many members to flee to southern neighbour Yemen, where they formed al Qaeda in the Arabian Peninsula (AQAP), which continued to threaten Saudi interests.

Charged with managing the country's borders, its internal crime-fighting apparatus and the internal intelligence force the mabahith, Nayef dismantled charities which used to collect donations for the late Osama bin Laden and his extremist network.

Nayef's son Prince Mohammed, who is the assistant interior minister and the kingdom's top counter-terrorism official, escaped assassination in 2009 when a suicide bomber from Yemen tried to kill him.

In recent years he transferred day-to-day security responsibilities to Mohammed, who has been even more methodical in pursuing Islamist radicals and battling their ideology.

But critics accused Prince Nayef of targeting democracy and human rights activists while neglecting, until recent years, the rise of Islamic radicalism in the country.

Saudis, however, showed support and appreciation for the strongman persona Nayef reflected because of public perceptions that he could deliver on national security.

Nayef told reporters early in 2009 that he opposed electing members of the consultative Shura council, or to include women in the group. "I don't see the need for that," he said.

He defended members of Saudi Arabia's religious police, who have often been accused of brutality and abuse.

As fall-out from the Arab Spring spread east from Tunisia, Libya and Egypt, the interior ministry played a key role in preventing popular demonstrations in Saudi Arabia.

Prince Nayef publicly thanked Saudis for not answering local activists' calls for protests.

The ministry also sought to maintain order in the face of increasing unrest in the primarily Shiite-inhabited and oil-rich Eastern province, where the kingdom accuses Iran of inciting the local population against the Sunni-led monarchy.

Nayef was the middle prince of the so-called Sudairi Seven, the formidable bloc of sons of King Abdul Aziz by a favourite wife, Princess Hassa al-Sudairi.

Among his other full brothers were King Fahd, who died in 2005, Crown Prince Sultan who died in 2011, and Riyadh Governor Prince Salman.