A top critic of President Vladimir Putin on Sunday faces a Kremlin-backed incumbent in a hotly contested Moscow mayoral poll, the first time an opposition leader has been allowed to stand in a high-profile election.
The candidacy of anti-corruption blogger Alexei Navalny has made the race the first genuinely competitive Russian election since the heady first post-Soviet years.
The vote will be seen as a crucial test of protest mood in a city which was shaken by huge demonstrations against Putin's decade-long rule in the winter of 2011-2012.
Moscow gave Putin, who sought a third presidential term, a relatively low 46.95 percent of the vote in the 2012 presidential election, well below the nationwide average.
Kremlin-backed Sobyanin, 55, is expected to win the Sunday poll with a majority, while Navalny is set to come second with around 20 percent, according to opinion polls.
A charismatic 37-year-old lawyer, Navalny shot to prominence during the anti-Putin rallies sparked by widespread claims of fraud in parliamentary polls.
Many ordinary Muscovites said they would vote for Navalny, who channels public anger against the Kremlin, even if some harbour reservations about his tough anti-migrant rhetoric.
"Moscow mayoral elections are much more than just Moscow mayoral elections," said Lilia Shevtsova, an analyst with the Carnegie Moscow Centre.
"The election should show us just how popular Putin is in Moscow, and therefore a mayoral election is turning into a key political factor."
The main intrigue in the upcoming polls is not how many vote for the pro-Kremlin incumbent but what happens to Navalny, who has been campaigning under the burden of a five-year prison sentence on what he says are trumped-up charges.
The blogger, who first made a name for himself exposing corruption among the elites, has vowed to jail Putin and his allies if he is one day elected president.
At the start of the campaign, Navalny was sentenced to five years in a penal colony on fraud charges and arrested in court.
A day later he was suddenly released pending appeal of his term, in an unprecedented move observers say showed the Kremlin did not know how to handle him.
Despite Navalny becoming an increasingly visible presence in Russia's politics, Putin still refuses to mention him by name and refers to him as "this gentleman."
The Russian president made no secret of his support for his former chief of staff Sobyanin, praising him profusely in an interview ahead of the poll.
"He speaks less and does more," Putin told state Channel One television. "I love such people."
Sobyanin was first appointed to the post in 2010 after then mayor Yury Luzhkov fell out with the Kremlin.
Earlier this year he called early elections in a move analysts said was designed to catch the opposition off guard and retain Kremlin's control in the Russian capital as economic trouble looms ahead.
Many say he's done a lot for Moscow over the past few years and will be rewarded at the ballot box.
Throughout the campaign the buttoned-up Kremlin functionary has avoided overt political rhetoric and shunned television debates, instead focusing on sprucing up the city of 12 million.
By contrast, Navalny made headlines with a Western-style political campaign mobilising the support of thousands of volunteers and securing more than 100 million rubles ($3 million) in donations.
Besides Sobyanin and Navalny, four other candidates are running in the polls but support for most of them is not significant.
Navalny charges that authorities are planning to rig the election, and threatened protests.
Analysts say Navalny's five-year term may be commuted to a suspended sentence if he performs well in the poll.
On Sunday, Russians will also elect local lawmakers and mayors in a number of cities including the Urals city of Yekaterinburg where an opposition activist is expected to run against a Kremlin-backed rival.