The decision by the district court was widely viewed as a move by President Vladimir V. Putin to eliminate his only viable rival in an election scheduled for March 2018.

Although Mr. Putin has been coy about his decision to run for a fourth term, there is little question that he will. Some observers had speculated that the president would want a weighty opponent to lend legitimacy to his next victory.

The best chance for a competitive race came to a screeching halt on Wednesday, however, when a district court in the small city of Kirov, nearly 500 miles northeast of Moscow, pronounced Mr. Navalny guilty of defrauding a state company. Russian law bars anyone with a criminal conviction from seeking elected office.

Since he first came to power in January 2000, Mr. Putin and his allies have gone to great lengths to silence or undermine all critical voices in Russia. It has been almost two years since the still-mysterious assassination of Boris Nemtsov, another charismatic opposition figure, on the doorstep of the Kremlin. His is the most recent in a string of killings of prominent critics — politicians, journalists and human rights activists — that remain unsolved.