Yuri Andropov, the former chief of the KGB security police, was named Communist Party leader today, succeeding the late Leonid Brezhnev as the most powerful political figure in the Soviet Union.

The selection was made at an extraordinary meeting of the Communist Party Central Committee at the Kremlin this morning.

In making the announcement at 2 p.m., Moscow radio quoted the 68-year-old Andropov as vowing to devote all his "energy, knowledge and experience of life" to carrying out the domestic and foreign policy of his predecessor, who died Wednesday at age 75.

But, Andropov added, "We know full well that it is useless to beg peace from the imperialists. It can be upheld only by resting upon the invincible might of the Soviet armed forces."

Well-informed sources said Andropov's rise to the peak of authority came when the armed forces and key members of the Central Committee threw their weight behind his candidacy for the post of general secretary of the party, which had been held by Brezhnev since 1964.

In an effort to demonstrate stability and the cohesion of the new leadership, the government news agency Tass announced that Andropov was elected unanimously after being nominated by his principal rival in the ruling Politburo, Konstantin Chernenko.

The selection of Andropov came after months of internal maneuvering and factional struggle that followed Brezhnev's serious illness in March. Rivalry between Andropov and Chernenko, while it could not be observed outside the secrecy of the Politburo, was all too obvious among their supporters.

When it appeared, in the past couple of days, that Andropov was the logical politician to fill a power vacuum, rumors questioning his Russian background were spread, apparently by his opponents within the party. One of Andropov's grandparents was of Jewish origin, according to reliable sources. The Soviet Union has had a history of anti-Semitism.

But the balance of forces in the leadership reportedly was tipped decisively in Andropov's favor because the principal lobbies, including the armed forces, were disturbed by Chernenko's limited experience in foreign and security affairs.

Moreover, these elements have questioned Chernenko's general experience. He is widely regarded as a "faceless bureaucrat" who owed his rise to prominence entirely to his association with Brezhnev. He has long been the head of Brezhnev's personal office and effectively remained in that role even after being elected to the Politburo five years ago.

According to Soviet observers, Brezhnev's death was sudden and unexpected and caught Chernenko's supporters off guard. The official report, signed by a group of prominent physicians, also indicates that Brezhnev had suffered a sudden heart attack and that the medical team may not have been at his quarters when he died, for the report does not fix the time of death but says that his heart failed "between 8 and 9 o'clock" Wednesday morning.

The suddenness of Brezhnev's death was said to have left the field wide open for Andropov, especially once he secured the support of Marshal Dmitri Ustinov, the defense minister.

This morning, in a Byzantine ritual of power politics, Red Army troops along with security and militia forces formed an impenetrable ring around the center of Moscow, completely sealing off the Kremlin where the 320-member Central Committee was meeting.

With the exception of troops and military vehicles physically blocking the access to the Kremlin, the vast spaces of this imperial palace complex were completely empty.

After his election, Andropov led other Politburo members and senior officials to the Hall of Columns in the elegant House of Unions to pay respects before the body of Brezhnev lying in state.

The new leader kissed Brezhnev's wife Viktoria and her daughter Galina as he offered his condolences. Moscow television showed all other Politburo members, each wearing a red armband trimmed in black, following the same procedure.

In his speech, Andropov called on all Communists to close ranks and to do everything possible for the "good of the Soviet people and the triumph of communism."

"It is our prime duty to accomplish these tasks, to translate consistently into life the home and foreign policy course" set by Brezhnev, Andropov said.

Andropov made no reference to detente or any foreign policy issues, although he is regarded as a specialist on foreign affairs. He served as Moscow's ambassador to Budapest when Soviet troops crushed the 1956 Hungarian Revolution. He is possibly the best informed of the Soviet leaders on both foreign and domestic affairs, having served for 15 years as the KGB chief.

Western diplomats, while noting his almost militant references to Soviet armed forces, cautioned that it was much too soon to draw firm conclusions about the course of the new leadership under Andropov.

In his nominating speech, the 71-year-old Chernenko described Andropov as a "selfless Communist" respecting the opinions of others and a colleague who had absorbed "Brezhnev's style of leadership." Chernenko also emphasized Andropov's experience in ideology and foreign and domestic affairs.

But Chernenko stressed that it was "now twice, thrice more important to conduct matters in the party collectively." The remarks and the tone of his speech suggested that he and his supporters are expecting Andropov to stick to collective leadership patterns developed under Brezhnev.

None of the speeches or other proceedings in the Kremlin this morning were shown on Moscow television, and the only television pictures of the leaders filing past the flower-bedecked bier of Brezhnev showed Chernenko visibly shaken.

Many of the Brezhnev loyalists in the upper reaches of the party and government hierarchy had hoped that Chernenko, as the closest aide of the dead leader, would quickly step into his patron's shoes. Chernenko had in effect filled in for the ailing Brezhnev for the greater part of this year and had control of the party apparat.

Andropov made his move to reach the top earlier this year following the death of Mikhail Suslov, the chief ideologist, last January and Brezhnev's illness in March. After a long career in the Central Committee apparat -- he was one of its secretaries from 1962 to 1967 -- and his tenure as head of the KGB, he moved decisively to take over the post vacated by Suslov.

He was selected to replace Suslov in April, while Brezhnev was convalescing, and this was formally confirmed by the Central Committee in May.

But as Brezhnev's health improved so did the fortunes of Chernenko, who had become particularly active and who recently made a foreign policy speech, something that was not his forte. It was widely speculated here that Chernenko's chances to consolidate his power were greater the longer Brezhnev lived. The late Soviet leader clearly favored him.

Moreover, Andropov's long association with the KGB and his role in suppressing dissent were looked upon as drawbacks in his pursuit of the highest office. In all his recent actions he appeared to have deliberately sought to dissociate himself from the KGB and has been portrayed by his backers as a modern, sophisticated and moderate politician.

Despite the surface unanimity on all issues, Chernenko and Andropov had taken different positions on one of the main domestic issues -- the economy. Chernenko has been closely identified with the government's main domestic initiative this year, to improve agriculture. He has been a vigorous advocate of the so-called "food program."

Andropov, on the other hand, is one of the few Kremlin leaders who never publicly mentioned or endorsed the program of new large capital investments in agriculture to improve its production. His silence on this issue even during a major Kremlin speech last May clearly indicated his reservations.

It remains to be seen whether his opposition to the traditional way of coping with a disastrous situation by throwing more money at it reflects a more profound realization that radical adjustments are needed in Soviet agriculture.

There has been a growing feeling among the elite here in recent months that new vigor and fresh ideas were needed to deal with the economic crisis. His supporters are cultivating an impression of Andropov as a pragmatic and clever politician and intellectual who is not afraid of new ideas, but whether this is a part of the image building or the real thing could not be determined at this stage.

It is expected that his elevation would induce some changes soon, but speculation mainly focuses on personnel changes. No successor has yet been announced to Brezhnev in his role as chief of state, a largely honorific post. It is expected that this question will be resolved when the Supreme Soviet meets Nov. 23. The vacancy gives Andropov an opportunity to remove potential rivals or opponents in a graceful way.

On foreign policy matters ranging from Afghanistan to Poland to East-West relations, it is expected that the forthcoming funeral of Brezhnev and the presence here of numerous foreign leaders and officials would provide some opportunities for discussion.

Although he is described as an exceptionally knowledgeable and cultured man, Andropov has never visited a Western country. His views on the United States are not known nor could his intentions be concluded from today's strident remarks.

But he will have an opportunity to meet with Vice President George Bush, who once headed the CIA and could possibly establish rapport with the former KGB chief, and Secretary of State George P. Shultz. Diplomats here believe that the meeting could provide an opportunity for Washington to begin an easing of tensions with Moscow.

Given his record and his intimate ties with the two key elements in the establishment -- the KGB and the armed forces -- it is expected that Andropov will be able to consolidate his authority in a relatively short time, according to many observers.

With such backing and his hard-line reputation, according to this point of view, Andropov may turn out to be far more flexible and decisive than most of his colleagues.

It is known, however, that his health is rather poor. His eyesight had weakened, and he appeared physically frail at the last Kremlin reception.