More than two weeks after the imposition of martial law in Poland, Marek Kowalczyk still cannot believe what has happened. Wearing a faded Solidarity T-shirt, he sits at home in Warsaw brooding over where the Polish revolution went wrong and what lies ahead.

Bright and alert, 30-year-old Marek is typical of middle-level Solidarity activists who pinned their hopes on democratic changes. A short time ago, he believed that the union's roots in Polish society were so strong that it would prove impossible for even the most repressive regime to remove them. Today he sees those roots withering around him.

"If the Russians had come here, that would have been different; then, we would have known whom we were fighting against. But it's our own Army. We're confused and we don't know how to react," he complained.

Marek, a former journalist, was detained for several hours the day martial law was declared, Dec. 13, but released when he signed a routine declaration promising not to break the regulations.

His home is watched, and he is followed when he goes out. This effectively prevents him from taking part in Regular communications with Poland remain cut. Washington Post correspondent Michael Dobbs is in Warsaw. underground activities--even if he wished to do so. Like several other Poles connected with Solidarity who are still at liberty, he asked not to be quoted directly for fear of drawing attention to himself. The name Marek Kowalczyk is therefore a pseudonym.

"Openness was Solidarity's strength and weakness," he said. "It made us an easy target for repression, but it was also our strongest link with the masses. Now, however, our tactics must be different. To operate openly now is to land yourself in jail.

"For the time being, at any rate, conspiratorial work is all but useless since we don't even know against whom we should conspire," he said. "Out of every five people, you must assume that one is an informer."

The sense of suspicion--such a contrast to the feeling of unity and mutual trust that permeated Poland during Solidarity's emergence--has been cleverly exploited by the regime. The fact that many union activists like Marek were released after a short detention or not picked up at all has made them targets of suspicion to their former comrades.

Ironically, during the first hours of martial law, the security forces rather than the Solidarity activists seemed to feel more afraid. So taken by surprise were both sides that they thought there must have been some mistake.

Marek recalled that when he was taken to a Warsaw police station in the middle of the night, the police went out of their way to be polite. "They kept on telling us that they wanted to treat us well. It was as if they expected the tables to turn any minute."

A well-known writer--let's call him Ryszard--said he had received similar treatment during a week's detention. His police guards expressed sympathy for him and did their best to make life easier. After a freezing first night during which he could not sleep because it was impossible to shut the window, he was transferred to a more comfortable cell.

At one point, the director of the prison called him aside and whispered: "You don't know why you're here--and I don't know why I'm here guarding you. We just have to watch each other, so we may as well make the best of it."

Ryszard described the atmosphere in prison prior to his release as good-humored. It was like a never-ending university seminar, with the internees engaged in debates over Solidarity's tactical mistakes and the nature of the preparations for what was regarded as Gen. Wojciech Jaruzelski's military coup.

Inside the internment center, he said, it was assumed that the government would be able to ensure public order in the streets and put enough food in the shops to feed the population. After his release, however, he became more pessimistic about Poland's long-term chances of recovery.

"When I was in jail, I didn't realize the extent to which social life in Poland has distintegrated as a result of martial law," he said. "With telephones cut and travel banned, there's been a complete breakdown in communications. It will be very difficult to rebuild a normally functioning society."

For Poles of Ryszard's generation, people in their late fifties, martial law represents their third big disappointment. In 1956, they hoped for reform from within the Communist Party by supporting the return to power of the nationalist leader, Wladyslaw Gomulka. But, after a period of heady liberalization, Gomulka tightened controls.

In December 1970, they placed their hopes in workers who rioted along the Baltic Coast rather than accept increases in the price of food. The demonstrations were suppressed--proving that it was senseless to challenge a Communist regime on the streets.

This time, using a more sophisticated technique, Solidarity attempted to ignore the Communist Party altogether and create its own alternative institutions. But this experiment failed when it touched the central question of political power.

Ryszard concludes that it is not enough to change political institutions. "Each of us must change himself from within," he said. "The system cannot be repaired. We must once again think small, not big. We must start by cleansing ourselves and then unite in organizations capable of undertaking independent social initiatives."

Despite a generally gloomy outlook, most Solidarity supporters remain convinced that the experience of the past 16 months has changed Poland. The sight of tanks in the streets has proved once again that communism has failed, they say. An entire new generation has been infected with the spirit of resistance.

Marek commented: "Everybody is sitting at home, allowing their emotions to pile up. Sooner or later, they're bound to spill out on to the streets again."