The article is based on archival materials inherited by the Lithunian State from the Soviets. It concerns arresting the participants of Polish underground movement in the Vilnius District connected with the Service for Freedom of Poland - the Union of Armed Fighting - ZWZ. It is an attempt to present massive arrests of ZWZ commanders and members in the Vilnius District carried out in the spring of 1941. These arrests, prepared much earlier and precisely conducted, led practically to a complete stand still of the organisation activities for several months. According to estimations at that time, the arrests included about 100 people of the ZWZ Vilnius District. According to Soviet sources, in the period of March - May 1941 about 300 people accused or suspected of activities in ZWZ were arrested in NKVD prisons. The commanding and leadership cadre, mainly in Vilnius, suffered the greatest arrests. The ZWZ provincial structures were saved almost entirely and they in the future served as a perfect basis to reconstruct the organisation.