The Soviet Union yesterday welcomed the takeover of Cambodia by the Vietnamese-backed Cambodian National Front and denounced the ousted government of Pol Pot as "fascists" who were a "weapon... of ruling circles in Peking."

After watching the Chinese move from one success to another in their relations with such major powers as the United States and Japan, the Kremlin was clearly pleased by the stunning setback to Peking's allies in Phnom Penh.

The Soviet press gave considerable prominence to what it called the "liberation" of Phnom Penh, declaring that the pro-Vietnamese Cambodian Front "is the sole representative of the people of Kampuchea," or Cambodia.

Despite its avowedly Communist plolicies, the Pol Pot government never had good relations with Moscow. No Soviet diplomats were permitted to come to Phnom Penh after the Khmer Rouge Communists took over in April 1975. But until recently, the Kremlin refrained from calling for Pol Pot's ouster.

A month ago when the new rebel Front was organized under Vietnam's auspices, the Soviets gave it what amounted to recognition and predicted the speedy overthrow of the existing government.

Moscow's attitude has had little to do with Cambodia itself, although the Soviets did accuse the Pol Pot government of committing genocide against the Cambodian people, a theme also widely heard in Western countries.

The Soviets, analysts here believe, were more interested in improving relations with Vietnam, a close Moscow ally, and in besting China, the Kremlin's ideological enemy.

Since the defeat of U.S.-backed governments throughout Indochina, the Soviets and Chinese have been vying for influence in the region. Moscow has had a substantial edge from the outset in Vietnam and Laos. Now, it is assumed, the Kremlin will have considerable sway in Cambodia as well.

"This whole episode has been a plus for the Soviets," one senior Carter administration official said yesterday. "They've shown the world and their own people that the Chinese were impotent to help the Cambodians and they have boosted the Vietnamese."

The Soviets and Vietnam signed a 25-year friendship treaty in November at the close of a visit to Moscow by a top-level Hanoi delegation. Analysts here believe that the Soviets were informed at that time about plans for the Front and presumably also about the present offensive.

The Kremlin has supplied some arms to Vietnam, sources here say, but Hanoi has been well-supplied with captured U.S. war material left behind at the end of the last Indochina war. There is no credible evidence that the Soviets have been directly involved in the current fighting, despite Cambodian claims that they piloted jets attacking the country.

The Khmer Rouge, as Pol Pot and his colleagues were widely known in the West, had a curious relationship with the Kremlin from the outset. Throughout the war from 1970 to 1975, the Soviets allowed diplomats from the U.S.-supported Lon Nol government to remain in Moscow along with Khmer Rouge representatives.

When the Khmer Rouge took over in 1975, they ordered the few Soviets still in Phnom Penh to leave and bundled them on a truck along with non-communist foreigners.

The Soviets nonetheless tried to establish formal ties with the new government but never really succeeded. Only once in 1977 did the two communist governments exchange "fraternal" greetings and these, specialists recall, were notably cool.

As relations between Vietnam and China deteriorated in the aftermath of Hanol's victory, the Kremlin opted for the Vietnamese position on all major issues -- including Hanoi's enmity for the Khmer Rouge.

Senior analysts in Washington said yesterday that Moscow may find Vietnam a very independent-minded allyonce the situation in Indochina settles down. Despite Hanoi's preparations for a major assault on Cambodia, sources said, the Vietnamese turned down a Kremlin request for military base facilities at the former U.S. port in Camranh Bay that might have been used to send in Soviet supplies.

"The Vietnamese," commented one specialist, "take orders from no one."