Yuri Andropov, in stepping out of the shadows to become the Kremlin's new leader, has demonstrated exceptional skills in the vulnerable moment of succession. Yet, despite the apparent ease with which he moved into the top position, general secretary of the Communist Party, he acts as though he is fully aware that he has yet to gain control over the party bureaucracy that runs this vast country.

This is the view of well-informed observers, who say Andropov displayed great caution in his first two weeks in office and who forecast a period of change that would not unduly alarm party members.

The clearest indication of his caution came earlier this week when he refrained from taking the presidency, a largely ceremonial post, and instead became a member of the 38-person Presidium of the Supreme Soviet (legislature).

The Presidium, a sort of collective presidency with no significant authority, provides Andropov with the right to act as chief of state if he chooses. The Presidium is the standing body of the rubber-stamp Supreme Soviet, which meets twice a year, usually for two or three days.

Well-informed sources said Andropov, 68, did not want to take the presidency, one of the posts left vacant by the death of his predecessor, Leonid Brezhnev. Having served as chief of the KGB security police for 15 years until last May, Andropov did not want an image of a power-hungry man accumulating every title within his grasp, according to the sources.

His reluctance may have been based on awareness that he was elected party chief because he had superior political skill to offer at a moment of crisis rather than because he had a constituency within the party -- as did his principal rival, Konstantin Chernenko.

In the Soviet system, power comes with control over the 18 million Communist Party members, organized in 410,000 units that pervade all institutions throughout the land.

While the armed forces and the KGB are the two most powerful interest groups in Kremlin councils, the party bureaucracy remains the backbone of the system.

No one person can be expected to take charge quickly of such a vast organization even when -- as Andropov did -- he manages to gain control over the two principal bodies of party authority, the Politburo and the Secretariat of the Central Committee.

Although the Politburo and Secretariat are composed mostly of Brezhnev loyalists and thus would be inclined to back Chernenko, Brezhnev's closest associate, Andropov had several things going for him that tipped the balance in his favor.

With an apparent political paralysis in the Kremlin during the last months of Brezhnev's life, and as problems mounted at home and abroad, influential party leaders and the armed forces chiefs were looking for the most competent and decisive man to take over.

The struggle for power in the Kremlin involves not only personalities but policies for dealing with the economy and other issues. It is difficult to tell where the first stop and the second begin.

The party itself is a major issue. Its members have become accustomed to privileges, which in an economy of scarcities means access to food and commodities. It is estimated that about 6 million party members enjoy these privileges and draw high salaries without performing productive work.

When Nikita Khrushchev attempted to shake up this vast structure and introduce economic reforms, he was first sabotaged by the party bureacuracy and eventually removed.

Brezhnev, who replaced Khrushchev, attempted to talk the bureaucracy into adopting reforms. But change has been resisted for so long that it could trigger drastic consequences, such as removal of the leadership.

And yet, Andropov has inherited the leadership at a time the country seems to be heading into a serious economic and social crisis as well as challenge abroad. As ex-chief of the KGB, he must have a seasoned assessment of the scope of both domestic and foreign problems. The armed forces chiefs also have made it clear that they are concerned about the economic crisis in particular.

According to well-informed sources, Andropov and his backers observed all the rules of conspiracy following Brezhnev's death on Nov. 10 to ensure that the top post would not go to Chernenko or another Politburo member without solid experience in foreign and domestic affairs.

According to the sources, the military chiefs had made it clear to members of the leadership that they were behind the Andropov candidacy. Yet neither the military nor the KGB could revitalize the economy.

Andropov is believed to need time to gain the support of at least a large section of the party bureaucracy and to bring his people into top positions. In his first pronouncements, Andropov has insisted on continuity yet made it clear that he would like to restructure the economy.

Brezhnev had said since 1973 that changes in planning and management "cannot be delayed." But he never took forceful action in that direction, and during his stewardship party officials down to the lowest level seemed to enjoy tenure even when corrupt practices and mismanagement were exposed.

All clues seem to suggest under Andropov a gradual generational change with fresh blood and new ideas being let into the hardened arteries of Soviet society.

His background in the KGB also indicates a greater role for the secret police in the months to come. Various estimates put the number of the KGB employes at around 500,000.

Despite the stigma attached to it, the organization changed considerably during Andropov's 15 years at its helm. An ascetic man, he had eliminated criminal elements, hired well-educated people and reduced corruption.

Andropov is expected to turn to KGB operatives he knows for help and to bring some of them into the government. His first important appointment was of Gaidar Aliyev, 59, a career KGB officer and later leader of Azerbaijan, to become full Politburo member and first deputy premier.

Andropov's successor as KGB chief, Vitaly Fedorchuk, also was given unusual prominence at this week's Supreme Soviet session. He delivered a strident speech against Western subversion. Links between the two men are believed to date to the early 1950s, when Fedorchuk was working in Vienna and Andropov was serving in nearby Budapest as Soviet ambassador.

Fedorchuk, in a speech that took two-thirds of a page in the Communist Party's daily Pravda and other papers, described the KGB as being "in reality the party's fighting unit in defending the gains of the October Revolution."

Aliyev, who joined the security police at 19 and later was sent to run Soviet Azerbaijan, is reported to have cut the corruption and economic crime for which the republic had become notorious. Under his ruthless leadership, Azerbaijan achieved the highest growth rates in the country.

Andropov, in his expected struggle to push some economic reforms to make the economy work, may continue the anticorruption campaign. This also may take the form of a struggle against economic inefficiency and fraudulent statistics, as well as corruption.

Judging by his pronouncements, however, Andropov is likely to be a cautious reformer. He has indicated that he intends to focus on internal problems and that he may move toward flexible wages and prices and incentives to lift the limping economy. As a specialist on Hungary, he is likely to adopt some of its changes to open up the system.

But any meaningful reform would involve infringement on the privileges and the positions of the party bureaucracy. Soviet society moves slowly, and it takes a long time for party decisions to be translated into action. Frequently they simply vanish in the Byzantine bureaucracy.

Compared with his predecessor, Andropov has several advantages. He was elected in the prescribed political process. He has no need to settle accounts to gain new room for manueuver -- as did Brezhnev by denouncing Khrushchev, and the latter by denouncing his predecessor, Stalin.

In his pronouncements, Andropov has insisted that he will continue Brezhnev's policies as outlined at the last party congress. But Andropov's style has been sharply different from that of his predecessor, and he has already put his own imprint on the new leadership.

With his KGB reputation, he may be in the position to reassure the conservatives that he will not let reform get out of hand. That same reputation may reassure the reformers that he can be strong enough to handle the bureaucracy.

Another factor that could propel Andropov to use his authority more decisively -- beyond the dire state of the economy -- is the challenge posed by the Reagan administration with its confrontational foreign policy and arms buildup.

Despite all of this, Andropov is expected to move slowly, much in the way he almost imperceptibly positioned himself for the top job. His struggle with the party bureaucracy is to determine whether he is merely a transitional figure or if his political concepts are going to give a lasting imprint on what is already being called the Andropov era.