MOSCOW - President Dmitry Medvedev's affection for going online has turned off-color after an obscene characterization of a prominent opposition figure appeared on his Twitter account.

Medvedev's tweets are usually bland, such as congratulating victorious sports teams. So the post describing popular opposition figure Alexei Navalny as a sheep engaging in an unprintable and unlikely sexual act sparked huge attention.

A Kremlin statement says the message, a retweet of a posting by United Russia party ideologue and former parliament member Konstantin Rykov, was done by an unnamed member of the president's technical staff. It added "the guilty will be punished."

The post late Tuesday came on the second straight day of large demonstrations in Moscow and other cities by protesters over the vote fraud seen in Russia's parliamentary election Sunday.

The post does not mention Navalny by name, but clearly refers to him, denouncing the phrase the "party of crooks and thieves" that Navalny popularized on his blog to characterize United Russia, the country's dominant party.

United Russia, led by Prime Minister Vladimir Putin, won a slim majority of parliamentary seats in the election, far fewer than the powerful two-thirds majority it held in the previous State Duma. Russian and international observers say fraud and vote-count irregularities were widespread, and protesters say the fraud substantially inflated United Russia's count.

Navalny was arrested Monday on the first night of protests in Moscow and sentenced to 15 days in jail.

Rykov's Twitter account sarcastically includes a link to a video of Navalny, repeating the sheep characterization at the Moscow protest. The retweet has been removed from Medvedev's account.

Medvedev made a point soon after becoming president in 2008 of showing himself to be technologically astute, establishing both a Twitter account and a video blog. Despite his attempts to look modern, Medvedev is viewed by many Russians as earnest but largely ineffectual.