Russian security personnel stand guard Monday at the entrance to the Moscow apartment block where anti-corruption blogger and opposition activist Alexei Navalny lives. Russian police searched the homes of prominent opponents of President Vladimir Putin. (DENIS SINYAKOV/REUTERS)

Police raided the apartments of a dozen or more activists Monday morning, seizing documents, ordering opposition leaders to come in for questioning and raising apprehensions about how harshly authorities will handle a big protest march planned for Tuesday.

Ever since Vladimir Putin was inaugurated as president on May 7, Russians have nervously speculated about how he will deal with dissent. The raids Monday suggested that he will do as many predicted: make an example of a few to frighten the many.

Police targeted some of the most visible and charismatic leaders of the opposition, including Alexei Navalny, an anti-corruption blogger; Sergei Udaltsov and his wife, Anastasia, leaders of the communist-leaning Left Front; and Ilya Yashin, a 28-year-old leader in the liberal opposition. Ksenia Sobchak , a glamorous television personality long considered untouchable because she is the daughter of Putin’s mentor, was also raided. So were Udaltsov’s parents, Navalny’s in-laws and an aide to a confrontational member of parliament, Ilya Ponomarev.

Police told those who were searched that they had to appear for questioning Tuesday morning, most likely preventing them from joining the march and rally they organized, which is scheduled to begin at noon.

The raids came after the parliament hurriedly passed a law raising fines for protesters to as much as $48,000 for organizers of a rally at which laws are broken. Putin signed the law Friday, and it went into effect Saturday, just in time for Tuesday’s march.

The protest movement coalesced in December over anger at parliamentary elections that were described as rigged. Those elections were followed by March’s presidential vote, which put Putin back in the office for a six-year term. He had been prime minister after two earlier four-year terms as president.

The raids Monday immediately set off a firestorm on Twitter, where they were compared to Stalin’s purges of 1937. A hashtag that translates into English as “Hello 1937” quickly trended to the top of Russian Twitter — though these raids were far different than those of 1937, when henchmen knocked in the middle of the night and neighbors looked away. These searches were carried out with witnesses, including journalists summoned by Twitter alerts, although they began around 8 a.m. in the middle of a long weekend preceding Russia Day on Tuesday, a national holiday celebrating the declaration of sovereignty in 1990.

Navalny documented the search of his home, sending out cellphone photos of police searching his children’s room and confiscating a T-shirt bearing the slogan he popularized denigrating Putin’s United Russia party as the Party of Crooks and Thieves.

A spokesman for the Russian Investigative Committee said the searches were connected to a criminal probe of a demonstration May 6, on the eve of Putin’s inauguration, at which police and protesters clashed.

“In the course of the searches, a large amount of protest materials and literature with anti-state slogans, electronic databases and computers containing information relevant to the criminal case was seized,” the spokesman, Vladimir Markin, said in a statement. In addition, he said, euros and dollars amounting to more than $1 million, and distributed among more than 100 envelopes, were taken from Yashin and Sobchak. Lists of supporters were confiscated from Udaltsov.

Markin said Boris Nemtsov, a longtime opposition politician, was on the list but the search was delayed because he was hiding.

Protesters said the searches would only galvanize the undecided to join Tuesday’s march.

“Never mind,” tweeted Ilya Varlamov, a photographer and blogger. “Searches are good pr for tomorrow’s protest. This will bring out the undecided. And they won’t find anything anyway.”

Yevgenia Chirikova, an environmental activist, tweeted that people should come to the march to show that hard-fisted tactics are ineffective.

“We only have one opportunity to prevent 1937 — to show that it does not work!” she tweeted, urging her followers to join her at Tuesday’s rally for a free Russia.

Mark Galeotti, a professor at New York University who studies the Russian security forces, said the march Tuesday will be a pivotal event that will reveal much about Putin’s new interior minister, Vladimir Kolokoltsev, who he says is unlikely to question any orders from the Kremlin.

In a column in the Moscow News, Galeotti said police are prepared to wield a great deal of force, with a new water cannon and a pneumatic machine gun that can knock protesters down with plastic bullets, pepper spray or paint.

“What will they do?” Galeotti asked in his column. “Vladimir Kolokoltsev’s big day will tell us much about him and the Kremlin mood.”