MOSCOW -- In recent years, when public opinion turned against the Kremlin, there was one main way to find out about it -- the Levada Center, a respected group of sociologists who broke off their relationship with Vladimir V. Putin's administration and set up the country's only independent polling agency.
The Levada Center may have to shut down, its director announced on Monday, after prosecutors ruled that it "influences public opinion and therefore does not constitute research but political activity." In a letter, prosecutors said Levada could not continue to release its work without identifying itself as "a foreign agent," as required under a new law.
The leaders of many nonprofit organizations have vowed to defy the law. It requires groups to assume the foreign agent label -- which evokes treachery and cold war espionage -- if they receive financing from outside Russia and are deemed political in nature.
But Lev D. Gudkov, Levada's director, was far darker in his assessment Monday, saying just the distraction of fighting the requirement would effectively shut down the center, which pioneered American-style polling after the collapse of the Soviet Union.
"We are now observing the end of an epoch that began with Gorbachev's perestroika," he said. "These are simply the last remnants of freedom, freedom of scholarship and freedom of information. That's it -- it's ending with this. This prosecutor's warning is not just a single isolated act, and it's not just about us. It's the end of a 25-year period in Russia."
The prosecutor's letter, dated Wednesday, details a series of grants the center received from the MacArthur Foundation, the Ford Foundation and the Open Society Institute Foundation, amounting to around $800,000 over a three-year period. Mr. Gudkov said foreign sources accounted for no more than 3 percent of the center's funding.
The legal crackdown comes in an atmosphere of acute suspicion, after Mr. Putin accused foreign governments of using nonprofits to penetrate Russian society and spark unrest. Dozens of organizations are facing legal sanctions or warnings, ranging from the Baikal Environmental Wave to the Kostroma Soldiers' Mothers' Committee. The focus on pollsters, however, is new.
"If someone orders research, and pays money, then he also orders a certain sort of research, and gives a certain array of questions -- that's the point," said the lawmaker Yevgeny Fyodorov, asked about the Levada Center in an interview with the radio station Kommersant FM. "This is not important in and of itself, but as part of a machine of management -- actually the external management of the country."
Polling is important to Russia's leaders, who view popularity as a key stabilizing element in a system without competitive elections. The Kremlin spends lavishly on surveys of tens of thousands of Russians but often withholds the results, especially those that point to rising discontent. A division of the All-Russian Public Opinion Center, which works closely with the Kremlin, has reportedly received a similar warning that it falls under the foreign agent law.
The Levada Center's sociologists have clashed with the Kremlin for years. The center's founder, Yuri Levada, incurred Mr. Putin's wrath a decade ago by publishing polls that showed waning approval of the United Russia party and the Chechen wars. When Kremlin officials tried to assert control over his organization by appointing a new board of directors in 2003, Mr. Levada resigned and formed a private company, the Levada Center. His employees followed him.
Mr. Levada died in 2006, but his center's work has continued to serve a key measure of the mood in society. An April survey, for instance, found that 51 percent of Russians agree with a derisive nickname for United Russia, "the party of swindlers and thieves."
Levada's data often tells a different story from that of Kremlin-affiliated pollsters. For instance, Levada has reported that around 20 percent of Muscovites support Mr. Putin, far lower than the 64 percent found by a Kremlin-affiliated pollster that included only respondents who voted.
Losing that alternative viewpoint, Mr. Gudkov said on Monday, would set Russian politics back decades, "like the Soviet time, when there was one newspaper, Pravda, and one television channel, something like that."
"A one-sided picture, and people believed it," he said. "That's the way propaganda works. You can't convince people that they are living well, but the idea that the United States has a hostile approach to Russia, that's easy."
This article originally appeared in The New York Times.