Poland cracks down on union, scuttles reforms; USSR keeps its distance from crisis next door

In one of the most severe and far-reaching internal crackdowns the Communist nations of East Europe have ever seen, Polish leader Wojciech Jaruzelski has clamped martial law on his troubled and restive country.

Clearly his move was meticulously prepared. He rolled it into place overnight Dec. 12-13 with well-oiled efficiency.

So much so that the Solidarity union movement seems to have been caught off balance. Although a number of its leaders were arrested, especially those considered ''extremists'' by the government, the rest of the union was slow to respond. A government spokesman said some detainees would be freed on a ''case to case basis . . . possibly very soon.''

With military rule imposed for the indefinite future, and with a curfew in place and a stream of regulations designed to control everything from the news media to opponents of all sorts, the three overriding issues are:

* Will the government's decision to have a showdown with the union sweep away the reforms painstakingly negotiated over the past 16 months? If the Polish Communist Party's hard-liners have their way, the nationwide hopes of last year's ''Polish August'' will go the way of Czechoslovakia's ''Prague spring'' of 1968.

There is still a chance, however, that by resorting to rule by a ''Military Council of National Salvation'' General Jaruzelski may be trying to curb union extremists in order to allow mutual restraint to prevail. At time of writing, for instance, Solidarity's moderate leader, Lech Walesa, was meeting with government officials in Warsaw to try to head off any general strike Dec. 14.

And in what is probably an effort to display evenhandedness, Jaruzelski also jailed some of the former Polish leaders blamed for today's economic crises such as former party chief Edward Gierek and Pyotr Jaroszewicz, his prime minister.

''The only hope is if this emergency decision opens up a chance in due course for the government to come to terms with Solidarity's moderates,'' said one of the shrewdest Western observers of the Polish scene. But considering that all trade union activity has been suspended and all social-political activity severely curtailed, any early prospect for such a development is far from bright.

* The immediate danger is that strike action by outraged Polish workers will thrust the country toward the abyss of civil conflict. Since the beginning of the month both sides have been ''digging in'' on ever more intransigent positions. It was probably the union's call Dec. 12 for a day of national protest Dec. 17 that triggered the government's decision to put the emergency measures in motion.

By late Sunday, troops, tanks, armored personnel carriers, and riot police were reported to have taken up positions in big cities and on main roads. Police were said to have used water cannon to disperse angry crowds outside Solidarity's Warsaw headquarters.

On the other side, workers in some plants were described as meeting to consider strike action, and one report referred to a leaflet calling for a general strike Dec. 14.

* If the confrontation inside Poland worsens, the Russians may eventually feel they have no alternative to the direct intervention from which they have shrunk so far. Incomplete reports suggested initially that the Soviet garrisons had not moved into action.

Quite apart from the cost in Polish-Soviet terms and the political implication with arms talks with the Americans only just begun, the Russians will still be hoping above all that this final contingency against total Polish breakdown can be avoided.

Invasion would open up a wholly new dimension. It would not only deal a crippling blow to the Warsaw Pact but also, in the longer term, to the stability and security of the whole communist political position throughout Eastern Europe.

Jaruzelski's 6 a.m. broadcast Sunday was delivered in his customarily quiet, unemotional tones. He appealed to Poles and the ''world at large'' to understand the decision was made in order to cope with a crisis in a country in the heart of Europe whose stability was vital to the whole continent.

The military were not taking over for good, he said, but only for as long as necessary to restore Poland to a normal situation. But none, he warned somberly , should count on weakness and indecision on the part of the government. The new regulations spell goodbye - for the foreseeable future - to the relaxed censorship legislated only a short while ago and a return to press and publishing controls as strict as those before the first reform movement of the middle 1950s. All regional radio and television has been suspended and staff replaced by special, party-vetted personnel.

The regime is taking no chances with the opposition. Besides appointing military delegates to take charge in towns, municipalities, and factories, the new ''military council of national salvation'' has placed the Army in charge of the ports, railways, and most power stations.