THE IDEA THAT the government should ever prohibit orderly, peaceful demonstrations in Lafayette Square seems, on its face, preposterous. That square has been the scene of many rowdy and unpopular demonstrations that have been tolerated because people in this country have a right to bring their grievances to the president's door. But this time there is a difference.

The Department of Justice made a compelling case before the U.S. Court of Appeals for closing the square temporarily to demonstrations concerning conditions in Tehran. The possibility is clear that the Iranians holding Americans hostage in the embassy there, not understanding what demonstrations in a democracy are all about, would interpret any demonstration so close to the White House as one sponsored by the government -- and base their actions toward their prisoners on totally erroneous assumptions.

While the possibility off so great a missunderstanding of American political activity might have seemed strange a few weeks ago -- pictures of anti-government demonstrations in front of the White House have circulated widely throughout the world in recent years -- it does not now. Recent actions in Tehran indicate an almost total lack of comprehension of American ideas and politics by both those holding the hostages and their leaders.

But even such a limited closing of one small part of this city to peaceful demonstrations should not be undertaken lightly. The Court of Appeals noted, quite rightly, that the availability of "other nearby sites" for demonstrations was a material factor in is decision to permit this ban.

Demonstrators for or against the shah and for or against Ayatollah Khomeini must be permitted to express their views in other parts of the city, so long as they conduct themselves peacefully. As tempting as it is to stifle any activity that might complicate the present situation, this country can no more afford to compromise its civil liberties than it can to bow to the demands of the Iranian mob.