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After the death of Boris Godunov, Prokopy and his brother Zakhary Lyapunov sided with False Dmitriy I. In early 1606, he took part in the Bolotnikov Uprising on the side of the rebels. In November 1606, Lyapunov came to Moscow and gave himself up to Vasili IV. After his repentance, Lyapunov was made a dvoryanin of the Boyar Duma (1607). In 1608–1610, Lyapunov led the army of service class people against a peasant uprising in the Ryazan region and supporters of False Dmitriy II. In July 1610, Lyapunov was the one to organize the overthrowing of Vasili IV.
When the Polish army occupied Moscow in 1611, Lyapunov, inspired by Patriarch Germogen's proclamations, levied the First People's Volunteer Army and became its leader. In March 1611, the people's volunteer army approached Moscow and blocked the invaders. In the summer of 1611, Lyapunov practically became the head of the interim government. On June 30, he issued a decree, which reinstated certain serf regulations and violated his own promises to Cossacks of "freedom and salary". Thereupon he was killed by the indignant Cossacks on August 1, 1611 O.S. The idea of a Volunteer Army was taken up by Dmitry Pozharsky, who finally delivered Moscow from the Poles in 1612.