MOSCOW -- Opposition leader Alexei Navalny swept up far more votes than expected Sunday while finishing second in Moscow's mayoral election, a pivotal contest that has energized Russia's small opposition in ways that could pose a risk to the Kremlin in the days and years ahead.
Even so, Mr. Navalny said he suspected that the vote count was inflated for the Kremlin-backed incumbent and he threatened to call his supporters out onto the street in protest today if the concerns were not addressed.
Nearly complete results released early today showed Mr. Navalny with more than 27 percent of the vote, while incumbent Sergei Sobyanin held a clear lead with about 51 percent, just enough to avoid a runoff. Exit polls, however, predicted Mr. Navalny would get as much as 32 percent.
As the results began to trickle out only two hours after the polls closed, Mr. Navalny said he suspected the vote count was being manipulated.
"We don't recognize the results that are currently being announced, and I would like to say that we won't give up one vote that we received," Mr. Navalny told reporters at his campaign headquarters late Sunday. "I call on the Kremlin and the mayor's office to restrain themselves from falsifications."
The election was being watched for what it bodes for the future of the opposition and for Mr. Navalny. He faces time in prison after being convicted of embezzlement in a case seen as part of a Kremlin effort to sideline him, but his strong showing could lead to a shortening of his five-year sentence.
Mr. Sobyanin needs more than 50 percent to win in the first round, but if he is seen as squeaking through unfairly because of vote-rigging, it could set off protests. It was reports of widespread fraud in a national parliamentary election in 2011 that triggered the unprecedented demonstrations against President Vladimir Putin's rule.
Mr. Navalny's campaign said its own exit polls showed Mr. Sobyanin below 50 percent. A separate vote count by observers also cast doubt on Mr. Sobyanin's clear majority.
Mr. Navalny said he suspected Mr. Sobyanin's results were boosted by falsifying the vote count of those who voted at home rather than at a polling station. He called for these votes to be annulled and for a second round to be held.
With ballots from about 90 percent of precincts counted, Mr. Sobyanin had about 51 percent and Mr. Navalny just over 27. The four other candidates trailed far behind.
Golos, Russia's leading independent election monitor, said the voting appeared to have gone smoothly, but there were fears that election officials would artificially increase the turnout to allow them to add votes for Mr. Sobyanin.
Sunday's mayoral election was the first since 2003 and the first since the Kremlin last year reversed Mr. Putin's 2004 decree abolishing direct elections for the Moscow mayor and other regional leaders.
Since Mr. Putin returned to the presidency for a third term, the Kremlin has cracked down on the opposition and tried to stifle dissent.