Complex sentence: On May 6 1987, Pamyat conducted an unregistered, and thus illegal, demonstration in the center of Moscow demanding an end to the construction of an officially sanctioned memorial project at Poklonnaya Hill.
Split into simpler sentences: On May 6 1987, Pamyat conducted an unregistered, and thus illegal, demonstration in the center of Moscow. They demanded an end to the construction of an officially sanctioned memorial project at Poklonnaya Hill.