The young prince already wielded huge power before he became heir, spearheading a sweeping economic and social reform programme for the kingdom.

Mohammed's youth is a novelty for a country that is used to ageing leaders -- King Salman is 81 and his predecessor King Abdullah died in 2015 aged around 90.

Saudi television channels showed the bearded Mohammed bin Salman kissing the hand of his cousin Mohammed bin Nayef and kneeling in front of the older prince, who patted his shoulder to congratulate him.

"I am going to rest now. May God help you," the former crown prince said, to which Mohammed replied: "May God help you. I will never do without your advice."

The king called on governors and other princes to pledge allegiance to his new heir and ordered a ceremony -- routine on such occasions -- after night prayers on Wednesday at a royal palace in Makkah.

A royal order said his appointment was endorsed by 31 of 34 members on the Council of Allegiance, which decides on succession issues.