Gaston Thorn, president of the Commission of the European Communities, warned in an interview this week that U.S. insensitivity to European economic and political dilemmas threatens to undermine the Reagan administration's attempts to reinforce Western security.

"There is no tension yet in European relations with the United States [because] the Reagan administration is enjoying a kind of political honeymoon with European opinion," Thorn said. "But there is anxiety. People are anxiously asking, 'When do you think the Reagan administration will start adapting its policies to help us? By Christmas? By next Easter?'"

Thorn, who is to meet President Reagan next week, said that European leaders need signals that Washington eventually will help relieve European economies from the unfavorable effect of such new U.S. policies as high interest rates, reduced eonomic assistance to developing nations and restrictions on trade with the Soviet Union.

His appeal for greater U.S. understanding, he said, reflected many European leaders' thoughts at a European Community summit last week, which discussed the Ottawa summit of industrial nations set for July 20 and 21. While striving to follow the Reagan administration and avoid the trans-Atlantic bickering that marked the Carter administration, European leaders now face domestic criticism because of perceived U.S. indifference to Europe's problems.

"The climate of Ottawa wil be important. No one expects miracles or big decisions, but we want a sign that the Reagan administration [is] starting to accept our grievances as legitimate . . .," Thorn said.

Numbers of Europeans, already divided over the more confrontational U.S. approach to the Soviet Union, are upset by the Reagan administration's unwillingness to compromise the ideology of its own domestic recovery program by talking about international economic measures that could revive European trade.

U.S. officials, in urging an expanded European military effort, may have underestimated the linkage in European politics between economic stability and military and political components of national security.

Thorn said, "There is an American way of saying, 'Now let's talk about defense' -- assuming that economic questions will be solved automatically by the free market. . . . Americans must understand that, whatever they do in the United States, it will be never be possible in Europe for politicians to dissociate defense from economic security."

The key topics on the Ottawa agenda -- North-South dialogue, East-West trade, Japan's trade role and cooperation among industrial nations -- have overwhelming economic dimensions for Europe. But the United States, a continent-sized market with its own resource base, tends to treat these questions as ideological issues.

In addition, high U.S. interest rates -- the Reagan administration's preferred way of purging domestic inflation -- have drained investment capital from Europe and driven up dollar rates, increasing the fuel and debt-service bills of developing countries that are major export markets for Europe. As a result, European industries have been unable to expand.

Amid the worst postwar economic crisis, he continued, "the United States generally seems to saying, "Wait until Uncle Sam is better, then we can look after your [European] sickness -- if you're still around to take the medicine."

Against a background of falling incomes and unemployment rising to traumatic levels, he warned about rising political instability in Europe. Thorn, an advocate of close trans-Atlantic ties, said that foes of the Atlantic alliance are exploiting the perception that the United States is indifferent to Europe's fate. U.S.-European friendship, he said, "hasn't been eroded yet, but we need to find a basis now for the period ahead."