IT HAD SEEMED that Argentina was emerging from its dark night of the 1970s, when its struggle against terrorism shaded into a frightful assault on its citizens' liberties, and so it is especially painful to see that some courageous Argentines who have tried to aid their unfortunate brethren have themselves run afoul of the security apparatus.

Over the weekend a single judge, using the court's own police, arrested without charges a number of prominent advocates of personal freedoms for Argentine citizens. One of the new prisoners is a man whose son is among the thousands of Argentines who "disappeared" at official hands and were presumably murdered in the earlier period. Another has a son who is still a prisoner and is a known torture victim. The judge also confiscated substantial files, including the documentation on 6,000 "disappeared" people. He is the same judge who ordered raids on human rights offices and the seizure of their files before a visit to Argentina by the Inter-American Human Rights Commission in 1979.

The leading theory is that the errant judge, conceivably acting alone but more probably acting for the still-substantial unreconstructed elements in Argentina's security forces and business community, became alarmed at the progress that the human rights cause was beginning to make in the courts. It is possible that this group realized that a more sympathetic approach to civil liberties, and to the possibility of transition to constitutional rule, was likely to come from Gen. Roberto Viola. He is to take over the government on March 29 with the first mostly civilian cabinet since the 1976 coup. Little or no taste is visible among the military for any coming to terms with the atrocities that it perpetrated and sponsored in the blackest years. But there is a view that the worst of Argentina's ordeal is behind it, and there is broad support for movement back to a more normal domestic life and for an end to the country's status as something of a pariah on account of its violations of liberties in the past.

For the moment at least, the Argentine government is hiding behind the contention that the arrests are the work of an independent judiciary. Given the well-documented record of judicial abuse and weakness in Argentina, however, this is a very tough proposition to sell. No Argentine regime can ask for the respect of people of good faith anywhere if it does not halt the persecution of the people newly arrested.

The first thought of many Americans, and not only Ronald Reagan's critics, was that Argentine hardliners had been emboldened by the new administration's downgrading of human rights. The State Department's nominee as human rights officer, for instance, is specifically on record with the statement -- which is false as well as cruel -- that the Carter administration's "scolding" of Argentina didn't really help the situation there at all.

The new administration can cite its stand at the United Nations Human Rights Commission in Geneva just the other day. The American delegate supported a resolution to keep the pressure on Argentina to account for past killings and disappearances, even while he lamented that other countries with similar or worse records were not being pursued. By yesterday, however, the State Department, saying privately that it was making inquiries in Buenos Aires, had not found public voice on the new arrests. The least it can do is to make plain that it disapproves of people's being locked up for trying to help the victims of an overweening state.