The site, Kremlin.ru, was unavailable for nearly an hour beginning at 11 a.m. The hacker group immediately claimed responsibility for the attack, claiming it was done in solidarity with the opposition, Interfax reported.
A representative of the Kremlin's internet service confirmed the attack and said that other government sites had experienced attacks as well, Interfax reported.
"We are very serious about these threats and are fully prepared," he said. "They are serious attacks, but the expertise of our professionals is not a joke."
On Saturday, the eve of the inauguration of President Vladimir Putin, the Russian branch of Anonymous posted a video message threatening to hack government sites, expressing support for the political opposition and stating that they would help protesters break "false government sites."
In March, hackers from the group previously attacked the sites of two regional branches of the United Russia party.