Complex sentence: The organizations that formed legitimized their work by referring to the principles enshrined in the Soviet constitution and, for the first time, to international agreements, and took particular care to emphasize the legality of their actions.
Split into simpler sentences: The organizations that nevertheless were formed legitimized their work by referring to the principles enshrined in the Soviet constitution and, for the first time, to international agreements. They attempted to operate within the existing legal framework of the Soviet Union, and the groups took particular care to emphasize the legality of their actions.