A special "quick reaction group" of four or five Politburo members is thought to be running the Kremlin's Afghanistan operation on a daily basis, with the full authority and agreement of ailing Soviet leader Leonid Brezhnev.

The members of this inner team, which may have been formed as long ago as last October, probably include Defense Minister Dmitri Ustinov, Foreign Minister Andrei Gromyko, secret police chief Yuri Andropov, and candidate Poliburo member Boris Ponomaryov, a specialist in Soviet Communist Party relations with foreign communist governments and parties. Party dogmatist Mikhail Suslov is thought to be a close adviser, though perhaps not a regular participant.

This suggested picture of how the Kremlin is handling the situation it created for itself by pouring thousands of troops into Afghanistan last month, is based on interviews with knowledgeable foreign and Soviet sources.

No one here outside a tiny group of men can say with any degree of certainty precisely how the Soviet leadership is operating in this situation, which has brought Moscow worldwide condemnation, economic sanctions and a probable United States boycott of the 1980 Moscow Olympic games.This is because Moscow is preeminently a capital of secrets.

Yet, from what is known of Brezhnev's health and past Kremlin record in emergency situations, it is possible to make rough suggestions about who in the leadership is thought to be involved most closely.

Central to this scenario, which veteran foreign observers consider highly plausible, is the condition of Brezhnev. At 73, he has sharply limited powers of concentration, as Carter administration officials discovered during last June's Vienna summit, when Brezhnev seemed able to participate actively in discussions with President Carter and others not more than two or three hours a day.

Most sources who can claim any degree of knowledge about the way the Kremlin conducts itself are convinced Brezhnev approved the broad policy outlines of the Afghanistan venture and continues to do so. But they suggest he has a limited capacity to absorb and decide complex, detailed questions of policy and thus has been virtually incapable of involvement in the daily decision-making.

While these sources do not exclude the possibility that he has been led misleading information by advocates of invasion to strengthen their hand, they believe it highly unlikely -- contrary to some rumors in the West that he was presented with an ultimatum to invade, which he opposed and on which he was overruled.

As one foreign source long practiced in Kremlinology said, "Brezhnev is not a man who ever finds himself in an isolated position. He has ruled by consensus. If the opinion is to the right, he moves to the right. If left, he goes left, etc."

At the time, one source said he understood that "perhaps some decisions have been taken without Brezhnev's knowledge or direct approval." The source suggested that if this had occurred, it was in the nature of the inner group's need to deal quickly with a fast-moving and unpredictable situation. "He gave these people the right to decide," the source said with assurance.

The choice of Ustinov, 71, Gromyko, 70, Andropov, 65, and Ponomaryov, 75, to oversee the adventure seems logical from what is known of these men's longstanding ties to Brezhnev and their portfolios. All came into the Politburo in the 1970s under Brezhnev's direct patronage, and presumably with the backing of Stalinist-era ideologue Suslov, 77.

Ustinov as chief of the Defense Ministry, would naturally assume principal military operational responsibility, working through his Chief of Staff Nikolai Ogarkov.

Gromyko, foreign minister for more than two decades and a practiced "public" figure whose insights have obviously carried increasing weight since he became a full Politburo member in 1973, would predict foreign reaction to the intervention, and is assumed to be directing the Kremlin's overall handling of those troubles.

KGB chief Andropov is included because, as a source explained, "he is part of the organs of control. He and Ustinov guarantee Politburo power."

Ponomaryov, a dour-looking baldish apparatchik, has directed sensitive Kremlin relations with foreign communist government for more than 24 years. He would be in a position to advise the inner circle on how to construct the hybrid Marxist-Islamic governement in Kabul thought necessary here to ensure the survival of the new leader, Babrak Karmal. He would also aid in the attempt to defuse the anger and fear among Islamic countries over the Soviet military move against the Moslem insurgents in Agfhanistan.

Just outside may stand Suslov and Konstantin Chernenko, a 68-year-old politburo member who has been Brezhnev's most powerful personal aide, directing the flow of paper to his desk and perhaps advising him privately on many matters. As longtime kingmaker with iron control over hard-line party theory, Suslov, spry but with weakened eyesight, is certain to have played a strong role in shaping the Afghanistan decisions. But he is reliably reported under medical orders to avoid a heavy workload.

Premier Alexei Kosygin, nearly 76, is said to be recovering from a heart attack and thus sidelined.

It is further assumed that working groups of the Central Committee and powerful Party Secretariat, assisted by such think tanks as the USA Institute and the Institute of World Economics and International Relations, function as policy-planning arms, as they do in most cases.

Ponomaryov would direct the effort to explain the invasion to foreign communist parties and Leonid Zamyatin, head of Central Committee information, would direct propaganda.