The failure of Soviet-mediated negotiations in South Yemen and the exodus of thousands of Soviet Bloc citizens from its beaches last week attest to a series of apparent miscalculations by the Soviet leadership under Mikhail Gorbachev, according to analysts here.

Government and academic analysts here and in other European capitals, including Moscow, look on what is happening in Aden as an embarrassment for Gorbachev in one of his first major foreign policy crises.

In addition, they say, the outcome of the civil war there may damage Gorbachev's efforts to broaden Soviet policy and relationships in the Middle East as a whole.

Where the Soviets went wrong, analysts here said, was in underestimating the political and tribal rivalries that remain in South Yemen despite nearly two decades of homogenizing allegiance to and dependence on Moscow. By permitting, and apparently encouraging, the return to South Yemen last year of Abdul Fattah Ismail -- the hard-line former president whose exile they had accepted five years earlier -- the Soviets, these analysts said, set the stage for conflict.

Some western diplomats say they believe the Soviets may have encouraged Ismail to challenge the control of Ali Nasser Mohammed, his former revolutionary colleague and successor as president. This minority view, shared by Bassma Kodmani Darwish, a Middle East analyst at the French Institute for International Relations, holds that the Soviets were less than happy with recent overtures by Mohammed toward moderate Arab states in the region, including Oman and Saudi Arabia.

But the predominant view among experts is that it was the Soviets themselves who encouraged Mohammed's effort.

"Politically, particularly since Gorbachev took over, the Russians have been looking for a broader policy in the Middle East," said a government analyst here. "Backing a single Marxist state in the area doesn't really give them the political influence they would like to have."

British sources say they have few doubts that, no matter who emerges the victor there, South Yemen will continue to be a loyal Soviet ally and an important strategic asset for Moscow. As the only Marxist state in the Arab world, it has been one of the most ardent disciples of Soviet political and educational structures and mirrors Soviet foreign policy.

More importantly, it provides the principle facilities for resupply and repair of the Soviet Indian Ocean fleet and the most extensive port between the Black Sea and the Soviet-run base at Cam Ranh Bay in Vietnam for Soviet vessels on their way to Vladivostok.

Soviet reconnaissance aircraft operating over the Persian Gulf and Indian Ocean are believed to operate from Yemeni bases, which also are believed to serve as Soviet electronic intelligence-gathering outposts.

Yemeni facilities also serve as refueling posts for aircraft such as the Antonov-22 transports used in the 1977 Soviet airlift to Ethiopia.

Although estimates vary, the Soviet presence in South Yemen is reported to be at least 2,000 military and civilian personnel, engaged in military training and construction and development projects. Other Warsaw Pact countries are well represented, with the East Germans in charge of intelligence and security services.

The price for the Soviet Bloc has been extensive military and economic aid that plays a large part in supporting the largely mountain and desert nation that most experts consider unable to support itself economically.

But British analysts, who look at South Yemen from the vantage point of 139 years of colonial domination as well as continuing commercial ties and the past three years of stable diplomatic relations, say the Soviets also had reason to know that Aden also can be an unpredictable and troublesome client.

Despite the Yemeni "commitment to 'scientific socialism,' " one said, "the relationship has been a pretty cautious one."

The analysts note that Aden appears to have turned a consistently deaf ear to numerous reported Soviet requests to establish autonomous military bases there. According to one source here, the Soviets, until last summer, had to go through the formalities of an official request each time a naval vessel pulled into Aden.

These analysts say they have seen no evidence to back up occasional western charges that the Soviets operate major air or naval facilities on the South Yemeni island of Socotra, 200 miles offshore.

The South Yemenis have shown occasional flashes of independence in other areas.

Fred Halliday, of the London School of Economics, said in the recently released "Soviet Interests in the Third World" that Aden has issued numerous proclamations of adherence to Arab national liberation movements, even when such policy contradicted that of the Soviet Union.

The ruling Socialist Party still is controlled by guerrilla leaders with an allegiance to pan-Arabism and strong tribal differences that often have reflected themselves in differing interpretations of the proper road to true socialism.

In recent years, Soviet commentators repeatedly have welcomed the relaxation of tensions between South Yemen and other states of the Arabian Peninsula, and made reference to Aden's more prosperous economic condition.

According to Halliday, Ismail, while considered a more ardent devotee of Soviet-style structural rigidity in the political system, appeared late in his tenure as president to have proven a poor administrator, worsening the already destitute economy.

Analysts said the Soviets appeared pleased with economic relaxations instituted under Mohammed. In the 1980s, Soviet trade with South Yemen has increased dramatically, and economic aid reportedly has doubled over the last five years.

Sources here said the Soviets began early last year to promote the integration of former rivals, such as Ismail, back into the government -- both as watchdogs for orthodoxy and as a way of smoothing over tribal and factional difficulties.

Analysts generally agree that, no matter how the trouble started following Ismail's return, it quickly got out of hand.

The Soviets, they said, overestimated the effect that late-coming "big-brotherly" intervention could have in calming the conflict once it started. As the battle goes on, the Soviets "are trying to be careful not to back the wrong side," said an official here. "They don't want to have made enemies when the fighting stops." But most analysts agreed that life would be easier for Moscow if Mohammed wins.