Saudi Arabia's new crown prince, Abdullah ibn Abdul-Aziz, has spent most of his government career running the kingdom's key internal security unit, the National Guard, and is likely to exert less control over policy making than did his predecessor, who became King Fahd yesterday.

Although Abdullah, 58, is known to be less of a supporter of close ties with the United States than Fahd, he had used American military help to build his tribally based Bedouin "White Army" into a modern military force that makes it a rival of the regular Army. His outlook, like that of the late King Khalid, is viewed as traditionalist in contrast with Fahd's backing for modernization.

Fahd as king will undoubtedly play a more dominant role in running the government than did the retiring, ailing Khalid. Moreover, Abdullah has a stammer that has prevented him from mastering the kind of oratory that Arab crowds expect from their leaders. This speech defect has been cited by diplomats and other observers who had once expressed doubt that Abdullah would be moved up to either the throne or the crown prince position.

The National Guard, commanded by Abdullah since 1963, protects Saudi Arabia's main cities, oil fields and communications facilities and is a deliberate counterweight to the Army.

The Army's commander, Prince Sultan, was the most likely candidate other than Abdullah to be named as the new crown prince. Sultan, however, is from the same Sudeiri tribe as Fahd and shares his relatively progressive outlook.

Abdullah has the loyalty of the Shammar tribes and, like Khalid, feels more comfortable sipping tea with fellow Bedouins or hunting with his falcons than governing. He is an expert horseman and maintains an extensive stable of Arabian thoroughbreds. His appointment was thus described as balancing that of Fahd.

Abdullah's reported anti-American tilt was evident in a dispute in 1976 with then-defense secretary James R. Schlesinger. Schlesinger had hinted at possible U.S. military moves against Arab countries if they mounted oil boycotts in the future, prompting Abdullah to criticize sharply the secretary and to caution against the "folly" of using armed force.

The new crown prince acted as mediator between Syria and Jordan when the neighbors nearly nearly went to war in December 1980.