Russian President Dmitry Medvedev — facing unprecedented voter protests — says he has ordered his government to investigate allegations of electoral fraud in last week's parliamentary elections.

Mr. Medvedev broke two days of official silence Sunday, revealing the probe to followers on the social networking site Facebook.

He said he disagreed with tens of thousands of protesters who took to the streets of Moscow and other Russian cities Saturday to demand a rerun of December 4 elections won by the ruling party. But he said demonstrators had the right to voice their views.

Neither the president nor Prime Minister Vladimir Putin has appeared in public in recent days, as protest organizers sought to harness opposition to the outcome of the polls. Critics accuse Mr. Putin's ruling United Russia party of complicity in widespread vote rigging and other irregularities. Former Soviet leader Mikhail Gorbachev last week called for election results to be annulled and a new vote held.

Saturday's rallies in Moscow, St. Petersburg and the Far Eastern cities of Khabarovsk and Vladivostok were largely peaceful. However, protests in Moscow and St. Petersburg earlier in the week triggered a massive police presence and the arrests of hundreds of demonstrators.

In comments early last week, U.S. Secretary of State Hillary Clinton said the elections were neither free nor fair. Prime Minister Putin promptly accused Clinton of meddling and fomenting unrest. He said Clinton voiced her conclusions before elections observers had released their findings.

Last month, Mr. Putin formally accepted his party's nomination to return to the presidency — a post that analysts have said he is certain to win. He announced his intentions in September, confirming a deal under which he would appoint President Medvedev as his prime minister.

The planned job swap has angered many in Russia, who said it would strengthen authoritarian rule and clear the way for Mr. Putin to become Russia's longest-serving leader since communist times.

If he regains the presidency, the 59-year-old Mr. Putin could serve two more 6-year terms and remain in power until 2024. He was first elected president in 2000.