Russian lawmakers gave final approval on Friday to a bill extending presidential terms, after a speech on the economy by Prime Minister Vladimir Putin revived speculation over his political ambitions.

The bill, which sailed through the State Duma, the lower house of parliament, would extend presidential terms from four to six years and would be the first changes to Russia's post-Soviet constitution adopted in 1993.

The reform was proposed little more than two weeks ago by President Dmitry Medvedev, who said it would help strengthen political stability, and has since been rushed through parliament despite a spiralling economic crisis.

Friday's vote was the third and final reading of the bill in the Duma, where 392 deputies voted in favour while only 57 voted against, the RIA Novosti and ITAR-TASS news agencies reported.

The legislation now needs to be approved by the upper house, where it is expected to pass easily, and by two-thirds of regional legislatures in a process that deputies estimate will take around a month.

The parliamentary session came a day after a congress of the ruling United Russia party in which Putin, its chairman, gave a wide-ranging speech on how the government would deal with the effects of the global financial crisis.

"Putin took the political initiative and responsibility" by giving a speech at the party congress "that was meant to be given by Medvedev," the Vedomosti daily reported Friday, citing political analyst Vladimir Badovsky.

The extension of presidential terms has been seen by some analysts as paving the way for Putin's return to the presidency after he left in May having served the maximum two consecutive terms allowed by the constitution.

Vedomosti earlier this month cited two Kremlin sources saying that Medvedev would resign soon after the approval of the amendments and that Putin would then run in a presidential election next year.

The constitution has no provision against former presidents who have already served two terms from running again, as long as some time passes between the end of their second term and their new candidacy.

Earlier this week, Putin's spokesman said the prime minister would continue his presidential tradition of holding a phone-in with the Russian people this year, further reinforcing the impression he is still the de facto leader.

The constitutional amendments passed Friday will also extend terms for the parliament from four to five years and increase parliamentary oversight over government in a move seen as bolstering Putin and his United Russia party.

Only members of the Communist Party, which has a minority in the parliament, voted against the changes on Friday. Communists have previously complained that the changes are part of an "authoritarian" trend in Russia.

Liberal parties critical of Putin are not represented in this parliament.

Kremlin critics held small protests outside the parliament building on Friday, with one man holding up a sign reading: "We Demand an Increase in Pensions, Not in Presidential Terms!"