It was late one afternoon in January when a squad of plainclothes police officers arrived at the headquarters of a prominent environmental group here. They brushed past the staff with barely a word and instead set upon the computers before carting them away. Taken were files that chronicled a generation's worth of efforts to protect the Siberian wilderness.

The group, Baikal Environmental Wave, was organizing protests against Prime Minister Vladimir V. Putin's decision to reopen a paper factory that had polluted nearby Lake Baikal, a natural wonder that by some estimates holds 20 percent of the world's fresh water.

Instead, the group fell victim to one of the authorities' newest tactics for quelling dissent: confiscating computers under the pretext of searching for pirated Microsoft software.

Across Russia, the security services have carried out dozens of similar raids against outspoken advocacy groups or opposition newspapers in recent years. Security officials say the inquiries reflect their concern about software piracy, which is rampant in Russia. Yet they rarely if ever carry out raids against advocacy groups or news organizations that back the government.

As the ploy grows common, the authorities are receiving key assistance from an unexpected partner: Microsoft itself. In politically tinged inquiries across Russia, lawyers retained by Microsoft have staunchly backed the police.

Interviews and a review of law enforcement documents show that in recent cases, Microsoft lawyers made statements describing the company as a victim and arguing that criminal charges should be pursued.

The lawyers rebuffed pleas by accused journalists and advocacy groups, including Baikal Wave, to refrain from working with the authorities. Baikal Wave, in fact, said it had purchased and installed legal Microsoft software specifically to deny the authorities an excuse to raid them. The group later asked Microsoft for help in fending off the police. "Microsoft did not want to help us, which would have been the right thing to do," said Marina Rikhvanova, a Baikal Environmental Wave co-chairwoman and one of Russia's best-known environmentalists. "They said these issues had to be handled by the security services."

Microsoft executives in Moscow and at the company's headquarters in Redmond, Wash., asserted that they did not initiate the inquiries and that they took part in them only because they were required to do so under Russian law.

After The New York Times presented its reporting to senior Microsoft officials, the company responded that it planned to tighten its oversight of its legal affairs in Russia. Human rights organizations in Russia have been pressing Microsoft to do so for months. The Moscow Helsinki Group sent a letter to Microsoft this year saying that the company was complicit in "the persecution of civil society activists."

Microsoft, like many American technology giants doing business in authoritarian countries, is often faced with ethical choices over government directives to help suppress dissent. In China, Microsoft has complied with censorship rules in operating its Web search service, preventing Chinese users from easily accessing banned information. Its archrival Google stopped following censorship regulations there, and scaled back its operations inside China's Internet firewall.

With pirated software prevalent in this country, it is not surprising that some of these groups might have some on their computers. Yet the issue, then, is why the police choose to focus on these particular targets - and whether they falsify evidence to make the charges more serious.

Microsoft also says it has a program in Russia to provide free and low-cost software to newspapers and advocacy groups so that they are in compliance with the law.

But the review of these cases indicates that the security services often seize computers whether or not they contain illegal software. The police immediately filed reports saying they had discovered such programs, before even examining the computers in detail. The police claims have in numerous instances been successfully discredited by defendants when the cases go before judges.

The plainclothes officers who descended upon the Baikal Wave headquarters said they were from the division that investigated commercial crime. But the environmentalists said they noticed at least one officer from the antiextremism department, which tracks opposition activists and had often conducted surveillance on the group.

The seized computers were not returned by the police until July, five months after they were removed. Their hard drives had been inspected by police experts in February. The environmentalists do not know whether all their data remain, and they are sure that files were copied.

Ms. Rikhvanova, one of the group's co-founders, who has been fighting to defend Lake Baikal since the 1960s, was unable to use her computer. When she got it back, she discovered that it had been disabled by a virus.