The world has been treated this week to the historic spectacle of the globe's leading communist power trying to transform itself into something less authoritarian.

In the process, Soviet society exposed a different face than the one it usually presents to outsiders.The debate over the rewriting of the Soviet constitution has brought to the surface a broad spectrum of disagreement ranging from dissent on the part of individual citizens to the nationalism that is aboil in many of the republics that make up the Soviet Union.

What a contrast to the usual image of the Soviet Union as stolid, monolithic, plodding, and drab.

Today, the Supreme Soviet or parliament approved by a 657-3 vote a package of almost 120 articles of legislation presented by President Mikhail Gorbachev as a step toward a political system based on law rather than decree from a central authority.

Among the articles are those that would replace the 1,500-member Supreme Soviet with a two-tier legislature equipped with veto power. There would also be a new electoral system, including an unlimited number of candidates for each seat in the congress, meet-the-candidate rallies, and televised debates.

Sound like western-style democracy? Up to a point, yes. But the similarities are sharply limited.

For one thing, the Communist Party would still call the shots. One-third of the seats in the new congress would be reserved for "social organizations," including the Communist Party and the official trade unions. This provision, together with the seats that party members are likely to win in regular elections, seem bound to assure Communist control over the new congress.

For another, it is still unclear whether or not rival political organizations would have the automatic right to nominate candidates for the congress.

Another departure from Free World standards is that the new Congress of People's Deputies is to delegate most of its day-to-day parliamentary business to a second chamber to be known as the Supreme Soviet. Members of this new Supreme Soviet are to be elected by the congress, not by direct popular ballot. The Supreme Soviet, in turn, is to elect a president to serve as head of state with expanded powers. It's hard to imagine Gorbachev not getting that post.

The new constitution has encountered opposition not only from those whose power would be curtailed, but also from those who fear the changes would further concentrate power and from Soviet republics seeking greater autonomy.

In assessing the reforms, keep in mind that they are being imposed from the top down rather than arising from the grassroots. Keep in mind, too, that even within its own borders, the Kremlin has a deeply-entrenched habit of saying one thing and doing something else.

As a case in point, the present Supreme Soviet has long been guaranteed wide powers under the constitution. But in practice the Supreme Soviet has functioned as a purely ceremonial body, meeting only twice a year to rubber-stamp decisions made behind closed doors by the Communist elite.

Though the new Soviet constitution sounds promising in some respects, many years will have to pass before it can be determined whether the changes are more than skin deep.