Primary resistance came from the Church and the Boyars. Moscow considered itself the "third Rome," and as a result was exceptionally orthodox. When Peter the Great insisted that Boyars shave their beards, the church protested vociferously; in fact the Metropolitan (head of the Orthodox Church) insisted that "God had a beard." The Boyars resented the loss of their power and authority. Peter's sister, who had been his regent, actually led a revolt by the Strelsky as a result of his attempts at modernization.
The second major reformer was Peter's Granddaughter in law, Catherine the Great, who considered herself an enlightened monarch, and attempted to rule accordingly. However, she was faced with a rebellion by Emilian Pugachev who claimed to be her dead husband (Peter III). After the rebellion was put down, Catherine decided that reforms were a mistake, and clamped down. By the time of her death, she left Russia in worse shape than it had been before she ruled.