Solidarity's transition to operations underground was a smooth one, and began as early as the night of December 12–13. Some of its activists—including very prominent ones, such as Zbigniew Bujak, Zbigniew Romaszewski, and Bogdan Lis—went into hiding that night and did not take part in the protests in factories and other workplaces, which of course by their very nature were not secret. The first larger group of activists in hiding was comprised of those who had taken part in the strikes but managed to avoid arrest after they had been broken. Many of these activists “went underground” immediately. Sometimes they did so as a group, such as in Białystok, where about ten people from the Regional Board first took refuge at the presbytery of St. Roch’s. Then, dressed in cassocks, they were taken to the parish church and only from there to their individual hiding places. The regional activists from Częstochowa attempted something similar, but failed. Six took refuge at the Jasna Góra sanctuary. The militia removed them brutally and unceremoniously, chasing away the pilgrims gathered there in the process and arresting many people inside the church. As the Paulists living there emphasized, not even the Gestapo did this during the occupation. Priests helped many union activists. In the first months, there were people in hiding in every Solidarity region. In some regions, the largest or most active, several dozen people or more were in hiding. Some of the activists from the Independent Students’ Union were also in hiding, as well as people from the pre-August opposition, and activists from the Independent Self-Governing Trade Union of Individual Farmers Solidarity (NSZZ IR). Sometimes, people whom the SB were not interested in went into hiding, too—which was understandable, considering the emotional atmosphere. Not surprisingly, people were apprehensive after seeing tanks on the streets, hearing rumors that the army had attacked striking enterprises, and learning that hundreds of people had been arrested.

Just how many people were active in the underground in a strict sense is difficult to establish. During the first months of martial law, probably hundreds of activists were in hiding. This number tended to wane, since although new people may have been joining—including people who had just been released from internment, or those who only became seriously involved in underground activities later—many more people were still being caught. There were also increasingly frequent cases of people “coming to the surface” for personal or family reasons, or because they were having difficulties staying in hiding.