COPENHAGEN, JULY 18 -- The European Community will strongly resist a revolutionary U.S. administration proposal to phase out all farm subsidies and agricultural trade barriers in the western world within 10 years, senior EC officials said today.

EC Vice President Frans Andriessen, speaking at a conference here attended by U.S. Agriculture Secretary Richard Lyng, said that the plan "goes further than is possible or desirable."

Andriessen, who also is the top agricultural official of the 12-nation EC, said that he had "a good deal of sympathy" for the free-market philosophy behind the American proposal. But he and other EC officials said that it would excessively disrupt world agricultural markets and draw the wrath of European farmers accustomed to substantial government assistance.

The EC wants to lower farm price supports and trade barriers gradually, but it cannot eliminate them altogether within any time frame, Andriessen and other EC officials said. They spoke at an EC-sponsored conference here on European-American relations.

The EC's position made clear that the American plan had no realistic hope of winning acceptance within the 93-nation General Agreement on Tariffs and Trade (GATT), which will consider it, EC officials and private farm analysts said. The EC hopes to present an alternative proposal in October at the GATT talks in Uruguay, Andriessen said.

Japan also has expressed strong reservations about the U.S. plan, so the Americans' only influential allies in the global trade negotiations are Australia, Canada and the other 12 nations in the so-called Cairns group of major agricultural nations in North America, South America, Europe, Asia and the South Pacific.

EC officials expressed doubt that either Congress or American farmers would be willing to give up all farm subsidies as President Reagan proposed on July 6. They suggested that the U.S. administration made the proposal primarily to put the EC on the defensive in the GATT talks.

"I do wonder how far congressmen will be ready to go with you when they have fully digested the implications for your own farm programs of what President Reagan is proposing," Andriessen said. "I do not think either side is really ready for it."

Lyng, trying to be conciliatory, expressed satisfaction with what he called the EC's "endorsement of some of our basic ideas." He said that the U.S. plan was "a proposal {and} not a demand."

But U.S. Agriculture Undersecretary for International Affairs Daniel Amstutz said that he was "disappointed" by the Europeans' statements. The EC was now taking a "tougher line" than it had immediately after the United States announced the plan, he said.

Amstutz expressed regret that the Europeans insisted on keeping subsidies that would be directly linked to agricultural production levels. The core of the American proposal is to "decouple" subsidies from output and thus discourage farmers from raising production in order to obtain more money from their governments.

Existing farm policies in the United States and the EC, by encouraging overproduction, have created enormous global farm surpluses of surplus dairy products, beef, barley, rye and olive oil that have depressed prices. The programs will cost the United States and the EC a total of more than $100 billion in direct and indirect subsidies this year.

EC officials conceded that they will have to reduce agricultural subsidies because the group's farm program is causing chronic budget deficits. The EC's farm program essentially is a victim of its own success, according to EC officials and private analysts.

The policy was devised 30 years ago, when Europe had to import much of its food. Today, the EC and the United States are the world's largest food exporters.

The EC currently is spending more than $1 million a day to store more than a million tons of surplus butter. It agreed earlier this month to sell 100,000 tons to the Soviet Union at a loss of more than 90 percent.

The EC countries with the most to lose from the U.S. proposal are West Germany and Ireland, EC officials said, because they have the largest concentrations of small, inefficient farms that survive only because of price supports.

A radical farm reform would force more than half of West Germany's 710,000 farms to shut down or be absorbed by larger farms, Rudolf Stoehr, chief economist of the Alfred C. Toepfer grain-trading firm in Hamburg, said.