Responding to reports of mistreatment in Poland's martial-law internment camps, a senior Polish official today declared that internees were being treated "in a humanitarian way" and termed reports to the contrary "totally a product of imagination."

In an interview published in the official daily, Rzeczypospolita, Justice Minister Sylwester Zawadzki provided a few details about how the thousands of internees live, eat and occasionally meet with outsiders.

Since the start of martial law Dec. 13, most of the leaders of the suspended independent trade union Solidarity, in addition to Polish intellectuals, writers, artists and others, have been held without formal charges.

Zawadzki said 1,300 of those interned have been released while 4,129 persons remain in detention. The figures corresponded roughly with the numbers provided by Gen. Wojciech Jaruzelski, Poland's military and Communist Party leader, in a speech to the Polish parliament last week, although the justice minister's figures were, inexplicably, somewhat lower. Jaruzelski had put the number released at 1,760 and those still interned at 4,569.

According to Solidarity sources, about 50 internment camps have been set up around the country. Conditions reportedly vary widely.

Internees are said to have been shuffled among the camps in an apparent effort to bring some order to a system that looked erratic at the start. Men are now being held separately from women; intellectuals who served as advisers to Solidarity are said to be grouped separately from workers; a dozen or so of Solidarity's top leaders are known to have been moved to a converted prison near Warsaw. Solidarity chairman Lech Walesa is isolated in a government-owned guest house in the Warsaw area.

Meantime, the Polish Army paper, Zolnierz Wolnosci, disclosed today that military courts have sentenced 245 civilians since Dec. 13 for martial law violations. The sentences have averaged from three years to seven years for offenses ranging from spreading Solidarity leaflets to organizing protests.

Officials of Poland's powerful Roman Catholic Church have visited a number of the internment camps, and the International Committee of the Red Cross has been permitted to send representatives to three such centers. None of the findings of these assessments have been made public, and unconfirmed reports persist of police beatings and harsh conditions.

One reliable source, just returned from a visit to a newly established internment camp in the northeastern city of Suwalki, today told of a beating in which 10 internees, ordered to strip naked in a snow-covered courtyard, were forced to run through a double row of police who beat the men with clubs. Called "the path of health," this is a well-known practice of Poland's security services.

The same source reported that women inmates in the northern town of Goldap had refused to see an International Red Cross team in protest of the fact that their camp had been selected as a model. The detention center, located in a luxurious recreation resort owned by Polish television and radio, is said to be favored by authorities as a showcase.

Zawadzki rejected stories of brutality against internees. "I would like to underline, with full responsibility for my words, that the internees are treated in a humanitarian way, and stories circulated by some Western agencies are totally a product of imagination," the justice minister said.

Zawadzki said his ministry retained responsibility for ruling on internees and for supervising the detention camps.

The minimum age for internment, he said, is 17. As for other criteria, Zawadzki said the internees were "only those toward whom there is justified suspicion that if left to remain at large, they wouldn't observe legal order or would carry on activity threatening the interests of defense or security of the state."

He said internees are receiving meals of 2,600 calories per day and some, on doctors' orders, are being given diet foods. An internee is entitled to two food parcels each month, each limited to 6 1/2 pounds. In addition, the internee can purchase food and tobacco worth 860 zlotys monthly (about $10.70 at the official exchange rate). In the first six weeks of martial law, Zawadzki said, those interned received 8,000 packages. Religious services--including 237 masses--have been permitted, he said. So have visits from family members and others, with the camp commandant's permission.

Zawadzki said no limits had been placed on correspondence for the internees.

The minister said there had been a wide assortment of complaints--about a lack of space in some camps, a shortage of heat, delays in mail deliveries, delays in the legal appeal process, the frequency of visitors and of religious services and the quality of health care.

But he claimed that complaints about what he termed the "improper attitude" of camp guards towards the internees were rare, and he said that the government was trying to improve conditions in the centers.

To support his picture of the camps, Zawadzki said that during the visits by church and Red Cross officials, no complaints of mistreatment were heard and the operations of the camps were not condemned. Church officials, however, have publicly attacked the existence of the internment centers.

Asked how much longer the internments would last, Zawadzki said, "It lies in the interest of the authorities for the period to be as short as possible."

Terming internment "not an act of vengeance but in essence a self-defensive reaction" of the state, the justice minister concluded: "I would like to stress that internment will cease to exist as an institution the moment martial law is lifted in Poland."