Sobyanin did not run as strongly as pollsters and administration officials expected.
Navalny's campaign has accused the mayor's office of falsifying the results. In particular, he zeroed in on the unusually high number of "voters from home" people too sick or old to make it to the polls. Navalny claimed many of them had been encouraged to cast their ballots despite not properly registering.
Golos, the independent election monitor, has supported Navalny's calls for a recount. The group said it saw no widespread vote-rigging in the city but had concerns about issues such as voting from home, which could have tipped a close race just enough in Sobyanin's favor.
Navalny, who still faces up to five years in prison on embezzlement charges, has urged his supporters to protest peacefully and legally against the election result.