Russian prosecutors opened a criminal investigation of former prime minister Mikhail Kasyanov on Monday over the purchase of a state-owned luxury riverside estate. Opposition leaders called it a politically motivated move against a critic of the Kremlin.

The General Prosecutor's office said it had seized false documents as part of the probe, launched after an ally of President Vladimir Putin accused Kasyanov of abusing his status while in office to allow him to later acquire a state-owned villa and 28 acres of land in Moscow. Prosecutors said they were looking into whether Kasyanov had legitimately bought the property.

Kasyanov, a potential candidate for the presidency in 2008 when Putin is due to step down, denied any wrongdoing.

"In all the years I have been in state service, I did not found any commercial organizations and I never owned any shares or any stakes in any firms," Kasyanov said in a statement read by his spokeswoman, Tatiana Razbash. His statement did not address specifically the alleged land purchase.

Kasyanov, 47, is currently outside Russia but plans to return later this month, Razbash said. Kasyanov rose to political prominence under Putin's predecessor, Boris Yeltsin. He was fired by Putin last year, weeks before the country's presidential elections.

Since leaving office, he has criticized Kremlin policies and the prosecution of Mikhail Khodorkovsky, the former co-owner and largest shareholder of Yukos Oil Co. Many saw Khodorkovsky's prosecution as punishment for his political ambitions.

Russia's ostensibly independent judicial system is widely regarded as being under the control of the Kremlin.

Politicians and analysts speculated that the criminal probe was a warning to Kasyanov to stay out of politics ahead of the 2008 elections. Although the constitution dictates that Putin cannot run for a third term, analysts say they expect that he will seek to ensure the election of a handpicked successor.

"The actions of law enforcement agencies in this case, as in many others, are politically motivated," said Grigory Yavlinsky, leader of the Yabloko party. Officials of United Russia, a pro-Putin bloc, said the probe shouldn't be seen as such.