Moscow's defeated mayoral candidate, Alexei Navalny, has launched hundreds of lawsuits contesting the victory of his rival, Sergei Sobyanin.
Mr Navalny delivered boxes full of complaints of fraud to Moscow city court, and began filing 951 lawsuits with 36 district courts.
The city court refused his request to stop Mr Sobyanin's inauguration on Thursday night.
The courts could take months to consider the lawsuits.
Mr Sobyanin, the Kremlin-backed candidate, won 51.3% of Sunday's vote - just above the 50% threshold needed to avoid a second-round ballot - according to Russian election officials.
Mr Navalny came second with 27.2%.
He called for a run-off and refused to recognise the results, saying they had been "deliberately falsified".
Mr Navalny said the count had been marred by "many serious violations".