On April 26, 1988, about 500 people participated in a march organized by the Ukrainian Cultural Club on Kiev's Khreschatyk Street to mark the second anniversary of the Chernobyl nuclear disaster, carrying placards with slogans like "Openness and Democracy to the End." Between May and June 1988, Ukrainian Catholics in western Ukraine celebrated the Millennium of Christianity in Kievan Rus' in secret by holding services in the forests of Buniv, Kalush, Hoshiv, and Zarvanytsia. On June 5, 1988, as the official celebrations of the Millennium were held in Moscow, the Ukrainian Cultural Club hosted its own observances in Kiev at the monument to St. Volodymyr the Great, the grand prince of Kievan Rus'.

What group was responsible for the April 25, 1988 protest?