Moscow - Vladimir Putin was inaugurated Monday to his third term as Russia's president, extending the former KGB officer's 12-year grip on power that most observers considered uninterrupted despite the past four years he served as prime minister.

In the course of a solemn and elaborate ceremony in the Kremlin Grand Palace, the 59-year-old leader was sworn into office under the 1993 version of the Russian Constitution, which was amended by outgoing President Dmitry Medvedev to increase the term of presidency in Russia to six years.

Putin then quickly submitted Medvedev's name to the parliament as his candidate to be prime minister, a move that had been expected.

In the presence of about 3,000 Russian officials and dignitaries and a few foreign guests - no leaders among them - Putin gave an oath "to safeguard the rights and freedoms of man and citizen, to observe and protect the Constitution of the Russian Federation, to protect the sovereignty and independence, the safety and integrity of the state, to loyally serve the people."

The center of Moscow, which was rocked by clashes between the anti-Putin demonstrators and police the day before, was quiet Monday morning. Riot police blocked thoroughfares and streets along the president-elect's route from the seat of the Russian government, the Russian White House, to the Kremlin.

Dozens of protesters who tried to stand on the sidewalk and cry "shame" to the passing motorcade were detained, among them opposition leader Boris Nemtsov, who had been arrested during Sunday's protests and fined 1,000 rubles (about $33) before being released.

Putin served as president from 2000 to 2008 before turning over the office to Medvedev for four years. Some experts expect that the third time around will not be as easy.

"We saw Putin in the past on the waves of his growing popularity, against the backdrop of the boosting market economy and the upward-headed oil revenues," said Dmitry Oreshkin, a senior political researcher with the Institute of Geography. "But we are just beginning to see Putin in the new, not so friendly surroundings, Putin losing popularity fast, Putin desperately clinging to power."

"He chose not to go while the going was good, and I am afraid that now that he can't afford the power slipping out of his hands, he will more and more rule by dictatorial methods, by discrediting, isolating and eliminating his political opponents and thus curbing the country's economic and political development."

But others were more optimistic about prospects for his third term.

"Putin will aim at overseeing a real breakthrough in economy, and we also expect to see a more competitive political environment with far greater discharges of political energy and hence a higher degree of political turbulence," Vyacheslav Nikonov, president of the Politika Foundation, a Moscow-based think tank, argued in an interview.

Nikonov hailed Medvedev's nomination as prime minister, calling him "the most experienced leader for this job."

The State Duma, the lower house of parliament, is expected to approve Medvedev's appointment today, opening the way for the formation of the new government.