Russia's President Dmitry Medvedev (R) and Prime Minister Vladimir Putin walk along tha stage to address the audience during the United Russia congress in Moscow September 24, 2011.(Agencies)

Russian Prime Minister Vladimir Putin has ended months of speculation by saying he will run for president next March in an election that could open the way for him to stay in power for 12 more years.

Putin's announcement was greeted by a standing ovationon Saturday at a congress of his ruling United Russia party, but it alarmed critics who say his return to the presidency could heraldan era of political and economic stagnation.

He and President Dmitry Medvedev have ruled in a power 'tandem' since Putin was forced by the constitution to yield the presidency in 2008 after serving a maximum two consecutive terms.

Putin, 58, accepted a proposal by Medvedev to return as president in the carefully choreographedcongress in a Moscow sports stadium.

"I want to say directly: (Medvedev and I) reached an agreement between ourselves long ago, several years ago, on what to do in the future, on who should do what," Putin said.

"But both I and Dmitry Anatolyevich Medvedev believe that this is far from being the most important thing -- who will do what, who will sit in what place. What is far more important is something else: how we will all work, what results we achieve, and what the citizens of our country think of this."

Over 11 years in power, Putin has cultivated the image of a vigorous leader and been filmed riding bare-chested on horseback, scubadiving and showing off his judoskills. His policies -- crushing a Chechen separatist rebellion, taming super-rich businessmen and bringing waywardregions to heel -- have similarly won him popularity among Russians.

Putin, described in leaked US diplomatic cables as the "Alpha dog" in the tandem, proposed his younger and more liberal protegereplace him as prime minister after the election to lead a young reformist government.