Allowing a civil action against Putin is out of character for Russia's Kremlin-loyal courts, where the big decisions are thought to be dictated from above.
Observers say it may be a carefully choreographed attempt to make the courts appear objective at a time when their reputation has been all but eroded. It is equally possible that the case will be used to silence Putin's opponents: His legal team could incriminate the plaintiffs, if it presents what it calls evidence to back up the prime minister's claims.
Putin alleged in December that a trio of opponents who were once in the government or parliament took billions from state coffers in the 1990s. He said former Deputy Prime Minister Boris Nemtsov, once-Deputy Energy Minister Vladimir Milov and former independent lawmaker Vladimir Ryzhkov had gone broke and were now seeking power again to fill their pockets.
Before Monday's court hearing Nemtsov, now the country's highest-profile opposition activist, told The Associated Press that he doesn't expect to win but said the fact the suit was even admitted is a victory.
Of the three, only Ryzhkov was absent from Savyolovsky District Court for the session. Putin had a lawyer present. The plaintiffs are demanding Putin retract the comments and pay 1 million rubles ($34,000) in compensation.
In the call-in show, Putin was asked what the three are pursuing.
"Money and power, what else do they want?" he said. "In their day they wrought havoc, in the 90s, (and) they stole quite a few billion along with the Berezovskys and others who are now in prison," he said, referring to Boris Berezovsky, the London-based tycoon who made his riches in the post-Soviet privatization period and fled in 2000 after falling out with Putin.
"They've been deprived of the hand that feeds them, they've gone broke and now they want to come back and fill their pockets. But I think if we let them do that, they won't stop at a few billion, and they'll sell out the whole of Russia."
Nemtsov is leading a new political party with Ryzhkov, Milov and host of other activists, including former Prime Minister Mikhail Kasyanov. In June primaries, the party, which is yet to be registered with the Justice Ministry, is to elect a candidate to run in the 2012 presidential election. Putin and President Dmitry Medvedev have said one of them will run.
Medvedev made an inauguration vow in 2008 to address what he called "legal nihilism," but there has been little change evident.
December saw the second conviction of Mikhail Khodorkovsky, formerly the country's richest man, on charges widely considered to be politically motivated.
Khodorkovsky, who funded opposition parties and negotiated with foreign companies over the sale of prized energy assets before his 2003 arrest, was effectively consigned to a second prison term when Putin suggested in the same call-in show that he belongs behind bars.
Nemtsov himself was arrested Dec. 31 after leaving an authorized anti-Putin demonstration and received 15 days in prison for participation in an unsanctioned rally.