MOSCOW (AP) -- Russia's only independent polling agency said Monday it may have to close after prosecutors targeted it for "political activity" under a law spearheading President Vladimir Putin's crackdown on civil society.

Levada Center published a letter, dated last week, from prosecutors who said its polls and publications are "aimed at shaping public opinion on government policy" and demanded it cease publication until it registers as a "foreign agent" under a law passed last year.

Russia has pushed strongly in recent months to enforce the law, which requires all foreign-funded NGOs that engage in ill-defined political activities to register as "foreign agents," a term that recalls Soviet-era propaganda casting suspicion on foreigners. The measure is seen as an attempt to restrict Kremlin critics and undermine their credibility.

Putin has long been suspicious of NGOs, especially those with American funding, which he has accused of being fronts for the U.S. government to meddle in Russia's political affairs.

Levada receives between 1.5 and 3 percent of its funding from foreign sources, including longtime bętes noires of Putin's foreign policy like the National Institute for Democracy and George Soros' Open Society Institute, according to center director Lev Gudkov.

Prosecutors and courts have cast the widest possible net in defining "political activity," including appealing to the European Court on Human Rights, almost any kind of advocacy work, publicizing rights violations, organizing a roundtable with a U.S. Embassy political officer, and environmental and gay rights activism.

Hundreds of groups have been raided since Putin gave a speech in February demanding the law be enforced. At least 44 NGOs have had legal actions brought against them, according to AGORA, a legal aid NGO.

The campaign has stoked public opinion against NGOs, according to a poll Levada released last week. Only 19 percent of Russians polled thought NGOs did any work of value, while more than half supported a total ban on all foreign funding for groups that criticize the government.