 Complete the following phrase:  The introduction of perestroika in 1987 made political activity possible again, starting an independence restoration process known as the Singing Revolution. The environmental Phosphorite War campaign became the first major protest movement against the central government. In 1988, new political movements appeared, such as the Popular Front of Estonia, which came to represent the moderate wing in the independence movement, and the more radical Estonian National Independence Party, which was the first non-communist party in the Soviet Union and demanded full restoration of independence. Reformist Vaino Väljas became the first secretary of the Estonian Communist Party, and under his leadership on 16 November 1988, the Estonian Supreme Soviet issued the Sovereignty Declaration, asserting the primacy of Estonian laws over Union laws. Over the next two years, almost all other Soviet Republics followed the Estonian lead, issuing similar declarations. On 23 August 1989, about 2 million Estonians, Latvians and Lithuanians participated in a mass demonstration, forming the Baltic Way human chain across the three republics. In 1990, the Congress of Estonia was formed as the representative body of Estonian citizens. In March 1991, a referendum was held where 78.4 of voters supported independence, and during the 