The Soviet Union, which recently concluded a defense consultation treaty with Vietnam, had no official comment today on the Chinese invasion of the Kremlin's ally. Instead, Soviet media merely repeated Hanoi reports of the fighting and Vietnamese calls for aid from Moscow, other socialist countries and the United Nations.

Senior Western diplomatic observers of Peking's military buildup along the Vietnamese border over the past week have maintained consistent skepticism that the Kremlin would intervene in any short-term border war between the two Asian powers.

Western observers here cautioned last night that there is no sure way to predict what the Soviets might do if the conflict continues for many days on a wide front.

The only officially sanctioned response here late last night appeared to be two demonstrations in front of the large Chinese Embassy in Lenin Hills, where about 500 persons rallied to shout denunciations of the Chinese. Witnesses said the picketers included hundreds of Asians carrying placards.

One source said the demonstrators appeared to come from nearby Moscow State University, where there are many Vietnamese students. A second, smaller demonstration occurred early Sunday morning. Police massed and closed off the street in front of the embassy to pedestrians.

The Chinese invasion poses a difficult and dangerous problem for the Kremlin, which must find a response that will back up the words of the November treaty of friendship and cooperation signed in November by Soviet President Leomd Brezhnev, Premier Alexei Kosygin and the Hanoi leadership, led by Communist Party Chairman Le Duan and Premier Pham Van Dong.

The treaty is not a formal military alliance between the two countries, but calls for immediate "mutual consultations" in the event of attack or threat of attack "for the purpose of removing that threat and taking appropriate effective measures to ensure... peace and security."

When it was signed Nov. 3, Brezhnev underscored the importance of the treaty in strengthening Soviet influence against its Peking adversary. He said the new accord "holds special significance at this complicated moment when the policy of the Chinese leadership has created new, major difficulties for social construction on Vietnamese soil."

In a direct challenge to Peking, Brezhnev said "The treaty already has become a political reality and whether they want it or not, they will have to reckon with this reality."

There is considerable Soviet strength aimed at China. According to Western defense analysts, the Soviets have about 44 divisions, six of them armored, deployed along the two nations' common border. Asian intelligence sources recently indicated that the Soviets are sending advanced armor to the sensitive Siberian and Trans-Baikal military regions, and the Soviet Siberian missile arsenal has been upgraded with medium-range rockets equipped with one-megaton warheads, as well as the most advanced Soviet mobile missile, the 4,000-mile range SS20.

The Soviet Pacific fleet based in Vladivostok is expected to be reinforced by a 40,000-ton Kiev class aircraft carrier, and Soviet Far East air forces include the most advanced Mig fighter aircraft.

The Soviets gave their Vietnamese allies substantial air support during Hanoi's successful blitz that toppled the Pol Pot government in Cambodia, and presumably could expand the effort to aid Hanoi in its newest struggle.

The invasion comes as Brezhnev apparently is vacationing and there is no way of knowing how the leadership will respond.

The Soviets, however, have made clear from press statements and contacts by senior official sources with Western journalists that the Kremlin is more deeply alarmed by Peking than at any time since the Chinese Communists split with Moscow in the early 1960s. Adding to deep-rooted Soviet fears of their 900-million strong neighbor has been Peking's successful year of diplomatic moves that has seen Chinese leaders go not only to Washington for normalization of relations, but also to such sensitive Balkan countries as Yugoslavia and Romania.