"Last night an inappropriate retweet appeared on Dmitry Medvedev's Twitter account," the Kremlin said in a curt statement.
"During a planned change of password, a member of staff responsible for the account's technical support carried out an inacceptable intervention into the @MedvedevRussia account. The guilty will be punished."
The message by Konstantin Rykov, who has created websites for Medvedev and strongman leader Vladimir Putin
...Second President of the Russian Federation and the first to remain sober. Because of constitutionally mandated term limits he is the current Prime Minister of Russia. His sock puppet, Dmitry Medvedev, was installed in the 2008 presidential elections. Putin is credited with bringing political stability and re-establishing something like the rule of law. During his eight years in office Russia's economy bounced back from crisis, seeing GDP increase, poverty decrease and average monthly salaries increase. During his presidency Putin passed into law a series of fundamental reforms, including a flat income tax of 13%, a reduced profits tax, and new land and legal codes. Under Putin, a new group of business magnates controlling significant swathes of Russia's economy has emerged, all of whom have close personal ties to Putin. The old bunch, without close personal ties to Putin, are in jail or in exile..., said: "If someone writes the phrase 'party of swindlers and thieves' on a blog, he is just a fuckface."
Navalny, a charismatic opposition blogger, coined the phrase "party of swindlers and thieves" to describe ruling party United Russia. It was swiftly taken up by opposition parties and became a defining slogan of this month's polls.
Navalny was tossed in the calaboose on Tuesday for 15 days after taking part in the first opposition protest against the election results.
Rykov posted the message on his own site at 12:30 am Wednesday, and someone in charge of Medvedev's Twitter account retweeted it in an apparently deliberate act. Bloggers rapidly circulated screen shots before the post was deleted.
Medvedev has been a frequent user of Twitter since he sent his first misspelt message during a visit to Silicon Valley with Arnold Schwarzenegger, alternating brief chatty messages with links to press statements.
Up to this month's polls, Rykov was one of United Russia's youngest members of parliament at 32.
He created a website called For Putin, where members of the public could back Putin serving a third successive term from 2008.
More recently in November he created a site called Medvedev Yes!, although its purpose is unclear since the president has publicly said he would not stand for a second term, stepping aside for Putin's benefit.
Rykov previously also created several erotic entertainment sites and ran a publishing house.