FILE PHOTO - Saudi Arabia's Deputy Crown Prince Mohammed bin Salman reacts upon his arrival at the Elysee Palace in Paris, France, June 24, 2015. REUTERS/Charles Platiau/File Photo

RIYADH (Reuters) - Saudi Arabia's King Salman left for holiday abroad on Monday and appointed his son, Crown Prince Mohammed bin Salman, to run the affairs of the world's top oil exporter in his absence, a royal decree said.

It is standard practice for Gulf leaders to hand over control to their deputy during travel abroad, but it is the first time Mohammed bin Salman, 32, has officially taken on the role since his sudden ascent to crown prince last month.

His promotion, replacing his 57-year-old cousin Prince Mohammed bin Nayef, ended two years of speculation about behind-the-scenes rivalry near the pinnacle of royal power.

The agency did not say where the king was traveling or how long he would be absent. But an Arab diplomat said he was expected to spend his holiday in Morocco.

The king's decree posted on Saudi state news agency SPA said the crown prince would be in charge of "managing state affairs and taking care of the interests of the people during the period of our absence from the Kingdom."

The movements of the 81-year-old monarch and other senior royals in the U.S.-allied, top oil exporter are closely watched because of their immense executive power.

Apart from his role as crown prince, Mohammed bin Salman also serves as defense minister and leads an ambitious reform agenda to end Saudi Arabia's reliance on oil.

(Writing by Sylvia Westall, editing by Sami Aboudi and Richard Balmforth)