Voters across Russia cast ballots Saturday on the second day of an election set to formalize six more years of power for President Vladimir Putin, who faces no serious challengers after crushing political dissent over his nearly quarter-century of rule.

The election comes against the backdrop of a ruthless crackdown that has stifled independent media and prominent rights groups. Putin's fiercest foe, Alexei Navalny, died in an Arctic prison in February, and other critics are either in jail or in exile.

The 71-year-old Putin faces three token rivals from Kremlin-friendly parties who have refrai...