Supporters of Russian opposition leader Alexei Navalny are to stage a protest rally in Moscow with the defeated mayoral candidate demanding a recount of the vote because, he said, it was rigged.
The Kremlin-backed candidate, Sergei Sobyanin, won 51.3% of the vote. Navalny came second with 27%, elections officials said.
Navalny, a vocal critic of President Vladimir Putin, claimed that the winning result was artificially inflated to pass the 50% threshold and avoid a run-off.
"We don't recognise the results that have been announced. They are rigged," Navalny wrote on his blog.
Moscow authorities gave permission for him to organise a 2,500-strong demonstration in the streets of the capital.
The 37-year-lawyer, who has led large protests against Putin, said he wanted to sit down with Sobyanin to organise a recount.
The Moscow Election Commission denied any violations of the count.
Sobyanin, 55, a former Kremlin chief of staff and Moscow's incumbent mayor, described the vote as the "most honest and open election" in the capital's history.
The turnout fell to 32%. The Communist candidate, Ivan Melnikov, came third with 10.7%.
Although he never had a real chance of seizing the mayoral office Navalny's result was surprisingly strong for an opposition leader. He rose to prominence on an anti-corruption campaign and led some of the largest anti-Putin protests of the past few years.
According to election officials 632,697 Muscovites voted for him - a much higher figure than the turnout of his past street rallies.
The mayoral election was the first since 2003 after the Kremlin reversed Putin's 2004 decree abolishing direct elections for the capital's mayor and other regional leaders.