IT WAS an odd spectacle in Berlin on Sunday.

There were tens of thousands of demonstrators, organized by the youth wings of the parties in West Germany's ruling coalition, throwing rocks and wielding axes and painting the Reagan administration as the enemy of peace for its nuclear policies. And there was also Secretary of State Haig, pleading that the United States not be held to a "supercritical standard" while the Soviet Union and its clients are given virtually a free pass in Afghanistan and Kampuchea. Mr. Haig was making a telling point. Or rather, the crowd was making it for him. Certainly the demonstration puts a burden on the leaders of Germany's "silent majority" to make clear to Americans that tactical disagreements are not sapping the fundamental Atlantic tie.

That was not all there was to brood on in Berlin. Getting specific, Secretary Haig observed that even as the United States is accused of delay on nuclear arms control, "others"--clearly he meant Moscow and its friends--seem to be violating the agreement signed in 1972 to ban biological weapons, including so-called toxins, which are poisonous chemicals produced by biological organisms. He referred to new findings, disclosed in greater detail at the State Department yesterday, suggesting that the deadly "yellow rain" visited on those struggling against communist invaders in Laos, Kampuchea and Afghanistan in recent years was an act of biological warfare. The findings center on high levels of potent mycotoxins, which are produced neither by indigenous organisms nor by any known facilities in those countries.

The track record of this and recent administrations makes it inevitable that these allegations will be treated skeptically in many quarters. This is especially so since the administration characterizes its own evidence ambiguously as "significant" but "preliminary." It is sending its material to various governments and to the experts who had already been assigned to investigate charges of chemical warfare at the United Nations.

This is fine as far as it goes. Chemical warfare is bad enough, but it would be unspeakable if the Soviet Union were actually conducting and sponsoring biological warfare. That would be violating an international commitment and setting a loathsome precedent. But why stop with passing on the evidence to those U.N. experts? They were set at work at a time when it was thought that the offense was chemical warfare. Now it is thought to be biological, and the biological warfare "convention" allows complaints to be taken to the more politically resonant Security Council. A common rap on arms control agreements is that they are not sufficiently enforceable. So why pass by the enforcement procedure of the biological warfare convention? Let American evidence--and Soviet conduct--be examined by experts and displayed in a political forum as well.