One demonstrator, Hugh DeWitt, a physicist at the Lawrence Livermore Laboratory in California, said he and other technical specialists here had concluded from seismic measurements that Tuesday's test was ''of extremely low yield, equivalent to less than five tons of TNT,'' far smaller than any other announced test.

''Either it completely fizzled or it was experimental in nature,'' he said.

Owen Chamberlain, a University of California professor and winner of the 1959 Nobel Prize for Physics, said he was puzzled not only that so small a weapon had been tested, but also that ''they had gone out of their way to make an announcement about it.'' Such a step, he suggested, was likely to goad the Soviet Union into ending its 18-month moratorium on nuclear testing. A Highly Organized Protest

Although today's protest scene at the Nevada Test Site was reminiscent of the 1960's, it was unlike the often disorganized, spontaneous peace demonstrations of that era and, indeed, was marketed with a promotional flair befitting Madison Avenue.

In recent weeks, news organizations have received numerous announcements and telephone calls about the event. Representatives of the sponsoring groups have appeared on network television shows in recent weeks, and today's arrests, which took place at 10 A.M., Pacific standard time, were timed to gain maximum publicity.

''Why do you think they scheduled the arrests at 10 o'clock?'' said Don Oliver, a correspondent for NBC News. ''So they could make the evening news.'' U.S. NUCLEAR TEST DEFENDED GENEVA, Feb. 5 (Special to The New York Times) - Kenneth L. Adelman, director of the United States Arms Control and Disarmament Agency, today defended the decision to conduct a nuclear test Tuesday despite warnings that the Soviet Union would end a self-imposed moratorium on nuclear testing with the first United States test of this year.

''If the Soviets want to resume tests, as I understand that they do, let them resume tests,'' he said.

Mr. Adelman also asserted that the timing of the test, which had initially been scheduled for today, was not connected with the start Tuesday of the 1987 session of the 40-nation Geneva Conference. Soviet officials implied that the test was ''cynically timed'' to coincide with the opening of the conference.

Soviet officials here echoed comments from Moscow that the Soviet Union would resume nuclear testing, but would only say that the tests would begin again at an ''appropriate time.''