A high-ranking United Russia party official said he believes liberal radio station Ekho Moskvy broke the law in publishing what he called an extremist letter by exiled tycoon and Kremlin critic Boris Berezovsky.

First deputy secretary of the United Russia presidium Sergei Zheleznyak said the letter "contains undisguised calls" to disrupt the presidential election and to forcibly overthrow the constitutional system, RIA-Novosti reported Thursday.

He said the content of the letter qualifies as extremist material, which media outlets are barred from publishing by Russian law, and that United Russia would ask the State Duma communications committee to investigate the matter.

The letter by Berezovsky, which was published on Ekho Moskvy's website on Sunday, accuses Prime Minister Vladimir Putin of breaking the rules of the Constitution by running for a third term as president and calls on Putin's election opponents to withdraw from the race.

Chief Ekho Moskvy editor Alexei Venediktov said he did not consider the letter to be extremist but that the website had taken it down in reaction to certain reader comments.

"I, honestly, did not see calls there to forcibly change the constitutional system, but, having read comments to the text, in which some radically inclined site visitors took it that way, I made the decision to remove it and give it to our lawyers for deeper analysis," Venediktov wrote on his blog on the station's site Wednesday.