BRUSSELS, JULY 1 -- Visitors to a "Euro-fair" here this week could feast on French oysters as an appetizer, Greek sausage as the main course, Italian pastry for dessert and Belgian beer to wash it all down.

The products illustrated the variety available from the world's largest trading bloc, the European Community, which started a new chapter today when a landmark act took effect that will make it significantly easier for the 12-nation group to pass joint legislation.

The EC, known to most Americans as the Common Market, also marks its 30th anniversary this year as the embodiment of a dream that most of Western Europe's democracies can gradually reduce trade and political barriers and form a kind of United States of Europe.

Two teen-age boys, trying to capitalize on this sentiment, hawked T-shirts at the fair reading, in English, "Born to be European." EC flags, each with 12 gold stars in a circle on a deep blue background, were in abundance.

But four straight years of EC farm spending deficits and sharpened disputes between the EC's richer members and its poorer, mostly Mediterranean states, have provoked rising concern that the EC is stuck in a financial and political morass.

Politicians and commentators have warned that the ideal of European unity is fading, as the EC has become associated in the public's mind primarily with the mountains of unsold butter and powdered milk that it has accumulated to bolster agricultural prices at consumers' expense.

Jacques Delors, president of the EC Commission, or executive body, has threatened to resign early next year unless substantial reforms are introduced.

"You can't get up every morning knowing that everything you're doing is to no avail," Delors said yesterday at the close of an EC summit.

The two-day conference offered fresh evidence that national interests often still supersede the widely felt desire to be good Europeans.

British Prime Minister Margaret Thatcher, in a rare rebuff to the tradition of European consensus, refused to join her partners in approving guidelines for halting the EC's slide toward insolvency. She said that the proposals did not go far enough to prevent overspending. "You can't go on pouring money into food you can't eat and that you have to pay to store," Thatcher said.

Moreover, the package accepted by the other 11 members concealed as many disagreements as it settled.

It left vague, for instance, how much money would be transferred to the EC's poorer members to help them catch up economically with their wealthier partners.

The so-called north-south dispute within the EC has become much more serious since Jan. 1, 1986, when the entry of Spain and Portugal boosted the strength of the less prosperous bloc consisting of the Mediterranean countries and Ireland.

The summit package also failed to spell out clearly how each member's future contributions to the EC budget would be calculated under a plan to increase the EC's total revenues by nearly 50 percent over the next five years. The increase in funds is considered essential to the effort to cap and eventually bring down the farm deficits, which currently gobble up 70 percent of EC expenditures.

The summit did manage to make modest cuts in this year's farm subsidies after West Germany yielded and accepted reductions in prices paid to its politically influential farmers. But the Bonn government is likely to make up some or all of the difference from its national budget, thus moving toward a renationalization of agriculture at the expense of European solidarity.

Finally, in what was perhaps the week's biggest disappointment, Spain vetoed a European airline deregulation bill because of Madrid's differences with London over whether the law should apply to a tiny airport at Gibraltar.

Spain does not recognize British sovereignty over Gibraltar, and Madrid feared that inclusion of the airport in the bill would help legitimize Britain's claim.

The veto killed a project that was put together in 18 months of laborious drafting. Now the work must start from scratch, because today's advent of the Single Europe Act means that new legislative procedures must be followed.

"Of course national interests must be safeguarded, but not in a way that the essence of the community is lost. . . . The vision of Europe is disappearing more and more," the West German conservative daily Die Welt said in a front-page commentary today.

The difficulties at the summit and over the air traffic bill soured what otherwise might have been an enthusiastic welcome for the Single Europe Act. It is the first major revision of the Treaty of Rome, which established the EC as a six-nation body in 1957.

The act's main provision is to eliminate the ability of a single country to blackball a proposal, as Spain did in the case of the airline bill. Instead, a complicated formula has been devised under which a majority of members can approve a bill.

The act allocates 76 votes among the EC's 12 members, with the more populous countries having more votes than the smaller ones. A proposal needs 54 votes to pass. In practice, opposition from two large countries plus one or two small ones is enough to block a bill.

There is a loophole. A single country still can veto a proposal if it says that its "vital national interests" are at stake. It must then present evidence to back up its claim.

The act also gives greater powers to the European Parliament, a popularly elected assembly that meets in Strasbourg, France. The parliament will now have two chances, instead of just one, to advise the member governments about a specific proposal. Ultimate responsibility still rests with the governments, however.

It is hoped that the act will help the EC in its task of creating a single, internal European market by 1992.

The EC already has eliminated customs barriers, but it now wants to do away with non-tariff barriers. These range from differing industrial products standards to restrictions on cross-border operations of banks and insurance companies.