A light breeze flowed up from the waterfront, cooling the crowds jammed into Havana's downtown streets for a rally supporting Vietnam's resistance to the Chinese invasion. It was, on the whole, a festive occasion and the excitement reached its peak with the appearance of a large, familiar figure, dressed in fatigues and sporting a splendid cigar.

"Fidel! Fidel!" the thousands chanted until Fidel Castro stepped forward to speak.

He hadn't planned to say anything, Castro began, since the rally was already scheduled to hear from other party functionaries. But he responded to the crowd's pleading and spoke for an hour, like a teacher to devoted pupils, about his view of China's perfidy.

What was so striking about this tableau to someone who has lived for a number of years in the Soviet Union and Eastern Europe was the apparently genuine rapport between Castro and his audience. It was a vivid contrast to the invariably staged reception which Leonid Brezhnev, for instance, gets on his rare public outings.

Soviet crowds are capable of outbursts of warmth for ballet dancers or poets. But I never saw a political personality in the U.S.S.R. get anything like the spontaneous response Fidel got at that rally. (Russians say the only leader they ever had who could arouse similar fervor was Stalin.)

Perhaps the most significant difference between Cuba and its Soviet bloc allies that I sensed is ideological zeal -- the impulse that assures popular support for shipping thousands of troops off to Africa (Havana teen-agers wear T-shirts with pictures of Angola's Agostinho Neto) and tolerance of serious shortages in consumer goods.

Cubans are proud that they are able to send doctors and teachers, as well as soldiers, to Angola and Ethiopia -- even if that means, as it does, sacrifices in their own development. There is a messianic quality to this, underwritten by Castro's exhortations.

This is, naturally, a limited conclusion, based on a week-long trip. But to a visitor steeped in the stale and generally cynical attitudes of political life in so much of Eastern Europe, Cubans seem to have a naive faith in the rightness of the state's goals.

In Cuba, Fidel and most of his comrades continue to call themselves revolutionaries after 20 years in power. And compared, say, to the graysuited apparatchiks of Moscow, Warsaw or Prague, they probably deserve the appellation. More importantly, the impression is that popular commitment to revolutionary principles -- among Cubans who have not fled to the mainland -- is still widespread.

CUBA IS by no means democratic in the sense that we regard the term. Yet there is an almost palpable atmosphere of public participation in matters great and small -- rather than, as there is in Czechoslovakia, a pervasive feeling of alienation.

Take the mobilizing of support for Vietnam against China. In his speech and other statement, Castro skillfully blended Hanoi's victory over the United States with his own long defiance of Yanquis and the new Yanqui courtship of China into an argument that made the interests of Cuba and distant Vietnam seem parallel.

In Moscow, the incantations on Vietnam's behalf might well be as vigorous, but Russians with their deep national chauvinism can't be made to identify so closely with lesser communist powers. Nor do Eastern Europeans have any spiritual bond to Third World Communists. Cubans, though, see themselves as leaders of the developing world, a power among the new nations, internationalists.

"We have shed our blood in Angola and Ethiopia," a banner at the Havana rally said. "We are prepared to do so for Vietnam." The record shows that on this score, Cubans mean what they say. The Vietnam campaign, with its posters and school recitations, was regimented, directed by the party. Still, it had an aura of sincerity.

ON A practical level, Cuba in the past few years has begun a system of representation known as Poder Popular (Peoples' Power). Municipal councils are chosen at neighborhood meetings, with no rigged requirement that members belong to the Communist Party, though the majority do.

In 1976, according to an American political scientist who studied the system, 2.8 candidates ran for each council seat. The balloting is in secret.

The elections involve meaningful although strictly local issues such as housing, road maintenance and jobs. Specialists say the councils make substantive spending decisions.

There are also provincial and national Peoples' Power councils in which half the members are chosen by electoral commissions from among those elected at the local level.

In theory, there is thus a flow of opinions and judgment up to the top. In reality, everyone seems to know that opposition to any fundamental government policy or too much freewheeling politicking is forbidden.

Still, nothing like this exists in the U.S.S.R., where elections are sterile, one-candidate affairs and all power rests in the bureaucracy. Even so limited a public voice in decision-making as Cuba's must stir improved morale and government responsiveness.

FOR ALL its distinctive features, Cuba is still very much like other countries in the Soviet bloc. Indeed, on some important points of style, the Cuban way seems more Soviet than that of some of Moscow's closer neighbors. It is hard to know if Castro has deliberately imitated the Kremlin or whether something about communist regimes makes them this way.

Housing projects in suburban Havana, for instance, look exactly like their counterparts in Soviet cities, with the same shoddiness that results from using prefabricated materials.

I made a list of things that reminded me strongly of Moscow, ranging from the grumpy service in stores and restaurants to dowdy clothing and the network of special stores where privileged people with access to hard currencies like dollars can buy hard-to-find and imported goods.

Especially noticeable is the similarity between the way the Soviets market vodka and the Cubans handle rum. In a food store on the outskirts of Havana, shelves were depressingly empty except for those with spirits, where a wide selection of expensive rums and other liquors were available.

Cubans are evidently willing to pay high prices for booze and do so, mopping up excess cash which might otherwise go unspent. The same phenomenon exists in the Soviet Union, with the same result: Alcoholism is a major social problem.

Spending a day with Ramon Castro, Fidel's older brother, a cattle breeder, was uncannily like a visit collective farms in southern Russia. There were smiling peasants in sparkling new houses and just a dash of fraud. "Have a drink," one beaming farmer offered, pulling out a bottle of export-only rum. He had been well primed.

OFFICAL CUBANS make no apologies for their closeness to the Soviet Union. On world affairs, they maintain that in key decisions of recent years, such as whether to send forces to Africa, the initiative was Cuba's. In fact, they say, the Soviets are uncomfortable at times with the forthrightness of Cuba's willingness to get directly involved across the seas.

As for modeling its society on the Soviet pattern, the party line is that 20 years of a U.S.-led economic embargo gave Cuba no choice but to develop ties with Eastern European nations. The Kremlin, everyone acknowledges, is paying increasingly massive subsidies to Cuba -- estimated now at about $4 million a day -- without which Cuba would be in desperate straits.

"We had to have help from someone," said a leading journalist. "Otherwise we would have ended up like the Communards [the revolutionaries who ruled Paris briefly in 1871], romantics with no viable system."

Cubans make a plausible case that the African adventures are of their own making -- an updating of Che Guevara's strategy for Latin America in the 1960s. Moreover, Cuba, like Vietnam, is a successful indigenous revolution (as opposed to the more conventionally political post-war takeovers in Eastern Europe). The presence of Soviet troops isn't a major factor in assuring the regime's stability.

In other words, despite the enormous Soviet influence on Cuba's foreign policy, the Kremlin is not in charge here. It would be very hard for Moscow to apply the overt military pressure on Cuba that was used when trouble erupted in Hungary and Czechoslovakia. To that extent, Cuba can, if it wishes, call its own shots.

But there is still the question of money. While it angers Cubans to be told so, Soviet backing is in keeping with Cuba's history of relying on powerful benefactors -- first Spain, then America, now the U.S.S.R. Adopting many trappings of its patrom seems to be what Cuba does, a bit like a copycat. Hence the Soviet feel to Havana.

Castro's bravado notwithstanding, Cuba has never been truly on its own, independent, nonaligned, in the way, for instance, Algeria is today.

Yet at bottom the foreign impact is probably just veneer -- a defense beneath which cubans can cling to their lovely, fertile island with its special character. Yes, Castro has brought his people firmly into the Soviet orbit. But he manages to make them believe it's for their own good.

Adopting many of the trappings of its patrons seems to be what Cuba does, a bit like a copycat .