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

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

On Monday, police targeted some of the most visible and charismatic leaders of the opposition, including Alexei Navalny, the anti-corruption blogger; Sergei Udaltsov and his wife Anastasia, leaders of the communist-leading 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 this morning, most likely preventing them from joining the march and rally they organized, which begins at noon.

The raids came after the Russian parliament hurriedly passed a law raising fines for protesters by 120 percent, to as much as $48,000 for organizers of a rally where laws are broken. Putin signed the law Friday and it went into effect Saturday, just in time for today’s march.