Navalny said Monday there was “clear falsification” of votes and Sobyanin’s majority is “guaranteed only by fraud.”
Kremlin-backed Sobyanin was widely expected to win. He is Russian President Vladimir Putin’s former chief of staff. Sobyanin was appointed mayor three years ago.
Navalny campaigned under the burden of a five-year prison sentence. The 37-year-old opposition leader, who has exposed alleged government corruption, says the charges are politically motivated and intended to silence him.
Navalny received his sentence in July, leaving the courtroom in handcuffs. A day later he was suddenly released, pending appeal.
Navalny, who was blocked from state-run television, conducted a Western-style campaign, mobilizing the support of thousands of volunteers.
By contrast, Sobyanin was all but invisible throughout the campaign. Instead, he focused on sprucing up the city of Moscow. Analysts say his campaign strategy was designed to remind voters of his access to the Kremlin and its money.