MOSCOW — Russia’s multiple arrests of the opposition leader Aleksei A. Navalny during street protests amount to a politically motivated campaign to silence him, Europe’s top human rights court ruled on Thursday, in a rare finding that a government had abused its prosecutorial powers with political intent.

Observers of the work of the European Court of Human Rights called the decision an embarrassment for the Russian government, as it was only the 11th ruling on abuse of such powers in the court’s nearly 60-year history.

Russia has for years faced a barrage of criticism over hard-line domestic politics under President Vladimir V. Putin, who has squelched independent news outlets and routinely jailed opponents.

But the ruling came at a delicate time in relations between the European Court of Human Rights and the Russian government, raising fears that in response to a decision vindicating an opponent of Mr. Putin’s, Russia could drop out of the treaty that formed the court.

Over the years, Mr. Navalny, a former real estate lawyer and Russia’s most prominent opposition politician, has been arrested dozens of times, usually under Russian laws against taking part in protests without a permit or in organizing them. Once, he was arrested while merely walking on a sidewalk.

The flurry of short detentions have ranged from a few days to weeks, and have kept Mr. Navalny out of public view before elections, avoiding the possible backlash at home and abroad that would most likely come from imprisoning him for a single, lengthy spell. The European court considered seven of Mr. Navalny’s dozens of arrests.

The ruling at an appeals level of the court, known as the Grand Chamber, found that Mr. Navalny’s arrests formed part of Russia’s “general move to bring the opposition under control.”

The court ordered the Russian government to pay Mr. Navalny 63,678 euros, or about $72,000, in compensation and legal fees and to alter its laws on public assembly to bring them into compliance with the European Convention on Human Rights. The court, based in Strasbourg, France, was founded in 1959 to enforce this postwar convention on European freedoms.

“I congratulate all normal people who welcome the victory of justice,” Mr. Navalny wrote in a post on his website after the ruling. The decision “means a lot not just for me, but for a huge number of people in Russia who are regularly grabbed and put in jail with obviously political motives.”

Mr. Navalny had won rulings against the Russian government over the same seven arrests in February 2017, when the court decided they were arbitrary, that he had not received fair trails and that his right to assembly had been violated.

The appeal handed him an additional victory for two of the seven arrests under an article that prohibits ulterior motives in prosecutions, including political motives, and that has an extraordinarily high bar of proof, lawyers who have litigated at the court said.

“My sense is the European Court of Human Rights has really done its job,” said Grigory V. Vaypan, a lawyer at the Institute of Law and Public Policy in Moscow. “For many people in Russia, the prosecutions of Navalny have looked political from the outset.”

The ruling comes at a tense moment, as Moscow, angry over previous rulings, has already threatened to withdraw from the court’s jurisdiction, ending a post-Cold War effort to integrate Russia into the Continent’s human rights architecture.

A judgment under the rule, Article 18, “essentially accuses the member state of lying” about the reasons for prosecutorial action, Jeffrey D. Kahn, a law professor at Southern Methodist University in Dallas and an authority on the European Court of Human Rights, said by telephone.

“That’s a pretty monumental decision,” he said, particularly as it was accompanied by an order to loosen laws on public assembly.

Russia has more cases before the court than any other country. In October, 10,950 allegations of rights abuse were pending against the Russian government, about 19 percent of the total docket. Russia has stopped paying dues for the court’s operations, and senior Russian officials say the country may soon sever ties.

“Europe is not only facing Brexit, one country leaving the European Union, but at the same time may also see a Ruxit, that Russia is leaving the European Convention on Human Rights,” Thorbjorn Jagland, the secretary general of the Council of Europe, said this month. “It will be a different Europe, and I don’t see any good things coming out of it.”

Follow Andrew E. Kramer on Twitter: @AndrewKramerNYT.

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