With their occupation of the U.S. Embassy now in its 19th week, the militant student captors are being portrayed by high Iranian officials as a divided, dispirited lot that has dwindled to a lonely nucleus of only 50.

Coming a week after the militants humiliated the government of President Abol Hassan Bani-Sadr by blocking a visit to the embassy by a U.N. commission, that portrayal may be a self-serving attempt to discredit the captors.

Regardless of their current numbers or internal divisions, the students generally have developed their political acumen over the months. They may not have learned much in the classroom, but their occupation of the embassy has provided an important lesson in domestic political science: how to hang on to power.

One remarkable fact about the occupation is that little is known after 4 1/2 months about the identity of the closed-lipped captors who once numbered about 400 and worked around the clock in three shifts.

At the International Court of Justice, the United States charged today that armed Iranian militants repeatedly threatened their American captives, including a woman, and kept them tied and blindfolded during "grueling interrogation" at the U.S. Embassy in Tehran, United Press International reported.

[The woman hostage, apparently among those released in late November, was interrogated by a man who pointed a pistol at her, told her one chamber was loaded and then repeatedly pulled the trigger to scare her.]

[The charges before the Internationed Court at the Hague were a continuation of the U.S. case against Iran.]

As a measure of the lack of knowledge surrounding the embassy takeover, Bani Sadr himself admitted in a recent interview, "We do not yet have, unfortunately, a strong enough intelligence service" to know what is really going on there inside the embassy.

The little the militants have said basically substantiates their insistence they are hard-line radical Moslems rather than the Marxists that the U.S. and Iranian governments have accused them of being.

If the reports of successive purges at the embassy are correct, the militants have been careful to weed out those whom they feared would embarrass them and cause their downfall.

The first purge is said to have taken place within days of the Nov. 4 occupation when leftists surfaced in a bid for power and were forced out.

A second group was reportedly eliminated when they applauded Bani-Sadr during his November visit to the embassy, in which he criticized the holding of diplomatic hostages.

Following Bani-Sadr's landslide Jan. 25 presidential election victory, his most outspoken enemies were kicked out for wanting to denounce him as a compromiser willing to settle the crisis with the United States.

The majority apparently reasoned they could ill afford to openly thumb their noses at such massive public backing.

Drawn from various faculties of Tehran's four major institutions of higher learning, the militants in many cases also had worked in the so-called Reconstruction Brigades formed after the February 1979 revolution to get the country moving again.

At different times, various political thinkers, clerics and politicians have moved in and out of the embassy.

Hojatoleslam Mohammad Moussavi Khoini was there from the first day and has served as a kind of chaplain. Ayatollah Ruhollah Khomeini's son Ahmad is a frequent visitor, and a dentist turned theoretician named Habibollah Peyman is said to wield considerable influence with some of the militants.

Basically, however, the militants seem to have managed their own affairs throughout and astonished Iranian politicians who were convinced at the outset that the crisis would be solved within a month.

Political sophistication has come slowly. For the first two months the militants benefited from the support of Khomeini, almost all the ruling Revolutionary Council and the Iranian public.

Their manipulation of the media -- especially American television -- seemed more expert than it actually was. Their line was simple. They demanded the return of Shah Mohammad Reza Pahlavi and his money and that the United States confess interference in Iranian affairs for the past 25 years.

However, they failed to understand that their tactics were helping to save the political fortunes of President Carter, whom they earnestly believed they were destroying. In a latter-day version of 1960s radical American student politics, they were convinced that their message would win over the American people who, in turn, would force the government to accede to their demands.

The militants have grown more cagey in the past two months now that their principal adversary is no longer the United States, but Bani-Sadr and his friends inside the Revolutionary Council who believe the occupation has fulfilled its purpose of humiliating the United States in the Iranian people's eyes.

But observers questioned the militants' political judgment in allowing recent television footage to be sent abroad showing press attache Barry Rosen in seeming serious mental distress.

Although they blocked a visit by the U.N. commission, its presence in Tehran obliged the students to allow a member of the Revolutionary Council, Ezzatollah Sahabi, to visit the hostages at length. His report destroyed much of the mystery that the captors had so successfully maintained until then. m

He reported he had seen all 50 hostages on the embassy grounds and that they were in good physical health. The hostages charged with being spies were relegated to the chancellery basement, still subjected to intermittent interrogation, were not allowed to speak and were kept in what amounted to solitary confinement, Sahabi said.

Lesser alleged offenders were kept on the ground and second floors, whose windows remained blacked out, he said. The most cooperative hostages reportedly enjoyed the relative comfort of ranch-style bungalows on the 27-acre compound.

The two women hostages -- Kathryn Koob and Elizabeth Ann Swift -- were guarded by women students clad in full-length black chadors.

The militants several weeks ago surprised many of their professors by showing up for examinations after months of absence.

Their main problem is doubtless what to do now that Khomeini himself has said that the hostage issue should be settled by the elected parliament. That may limit the militants' power.

Theirs is the somewhat desperate knowledge that once the hostages are released, they run the risk of being relegated to the garbage can of revolutionary politics by becoming just another of the many groups vying for attention here.

Like everything else that matters, that is a decision which depends on Khomeini. He is a leader torn between his revolutionary empathy for the captors and Bani-Sadr's warning that they are preventing the new Iranian society from taking root.

News services reported the following:

Roberts B. Owen, the U.S. State Department legal adviser, told the world court at The Hague that the 50 American hostages have been "confined like common criminals." He cited affidavits from 13 hostages released Nov. 20.

It was the first time in the 137-day embassy crisis that the United States had officially revealed some of the conditions the Americans have endured.

In Kuwait, meanwhile, Iranian Ambassador Ali Shams Ardakani, who recently met with Ayatollah Khomeini, said in a newpaper interview, "There will be no solution to the hostage question before May 15." He said the issue would not be the top priority for the new parliament, which is expected to convene in April.