Soviet Union national football team
Playing football Demchenko started out in his native Zaporizhia where his first coach (trainer) was Borys Zozulya and Viktor Vysochyn. When Demchenko turned 13, his new trainer was Anatoliy Vasyleha who coached him before Demchenko moved to Moscow. During his teenage years Demchenko was spotted by Gennadiy Kostylev, a Soviet Union national football team coach, and Demchenko participated in some international tournaments among junior teams. During dissolution of the Soviet Union, in 1990 Demchenko was invited as a prospect player to play for PFC CSKA Moscow where Kostylev also became the club's manager.