Russia's parliament was expected on Friday to approve constitutional changes extending presidential term lengths, a move opponents suspect could lead to Vladimir Putin returning to the presidency.

The State Duma announced a first reading would be held on constitutional changes "relating to changing the term of office of the Russian president and of the State Duma."

The changes, announced by President Dmitry Medvedev earlier this month, would extend presidential terms from four to six years and the Duma's mandate from four to five years.

Approval of the first ever changes to Russia's 1993 constitution requires the assent of both houses of parliament and of two-thirds of Russia's regional assemblies.

The Duma, which is overwhelmingly dominated by pro-Kremlin deputies, is expected to wave through the proposals without serious objection, despite criticism that the changes are being rushed.

Second and third readings on the legislation could take place next week, Alexander Moskalets, a deputy on the Duma's constitutional law committee, said on Thursday.

The changes have sparked fresh speculation that Medvedev, who succeeded Putin as president in May, could use the pretext of constitutional changes to step down early and make way for a return to the post by his mentor.

Both men have played down such claims without rejecting them outright.

Activists from the opposition Yabloko party mounted a small rally outside parliament, holding up a placard with a quote from Putin: "You should change brains, not the constitution," the Echo of Moscow radio station reported.

A deputy prime minister, Igor Shuvalov, has denied the changes are intended to strengthen the presidency, saying "it should be seen as a balanced model for a new political construct."

Predicting economic turmoil ahead for Russia, Vladimir Ryzhkov, a commentator and former opposition deputy, said in Friday's Moscow Times newspaper: "Putin might try to keep a step ahead of the game once again.

"Using pretexts such as the new constitution and the need to strengthen the state in the face of the crisis, he might through Medvedev announce snap presidential and parliamentary elections as early as March or April."

Voicing support on Wednesday for the changes, Putin insisted they had no "personal dimension."

He went on to say that: "As far as who could run for the next term or when -- it's too early to say" -- a response that raised eyebrows as it suggested doubt about the next scheduled presidential election date of 2012.

Medvedev was cagey when asked about his intentions in an interview with France's Le Figaro newspaper published on Thursday.

"I'm still working. Why are you trying to push me into some kind of decision? I can only say for sure that this new term can only apply to a presidential candidate who stands for the post after the passage of the corresponding changes," Medvedev said according to a Kremlin transcript.