* Leading Kremlin foes Navalny and Kasparov affected
* Sites accused of supporting illegal activity
* Move seen as part of crackdown on independent media
By Steve Gutterman
MOSCOW, March 13 (Reuters) - Russia blocked access to the internet sites of prominent Kremlin foes Alexei Navalny and Garry Kasparov on Thursday under a new law critics say is designed to silence dissent in President Vladimir Putin's third term.
The prosecutor general's office ordered Russian internet providers to block Navalny's blog, chess champion and Putin critic Kasparov's internet newspaper and two other sites, grani.ru and ej.ru, state regulator Roskomnadzor said.
The move was the latest evidence of what government opponents see as a crackdown on independent media and particularly the internet, a platform for dissenting views in a nation where state channels dominate the airwaves.
Ej.ru editor Alexander Ryklin called it "monstrous" and a "direct violation of all the principles of freedom of speech," radio station Ekho Moskvy reported. He said he did not know why the site was blocked.
The blockages will increase concern that Putin is seeking to tighten control over Russian society as he faces off against the United States and European Union in a bitter dispute over the future of Ukraine.
It came a day after the editor of independent news site Lenta.ru was dismissed after it received a warning over publication of remarks by a Ukrainian far-right leader in what dozens of its staff members alleged was Kremlin censorship.
"This is the latest political decision taken as part of the cleansing of the media space," Navalny's spokeswoman Anna Veduta said on Twitter. Another Twitter user called it "Black Thursday".
The Kremlin denies allegations of censorship or pressure on the media.
Roskomnadzor said Navalny's blog violated the conditions of house arrest recently imposed on the opposition leader, who is serving a five-year suspended sentence on a theft conviction he said was engineered by the Kremlin and may face another trial.
The other three sites were ordered blocked because they "contain calls for illegal activity and participation in mass events conducted in violation of the established order," the regulator said.
Putin, in power since 2000 as president or prime minister, won a six-year third Kremlin term in 2012 after weathering a series of protests led by Navalny and others and organised largely over the internet.
He signed a law late last year allowing prosecutors to order providers to block access to sites deemed to have published calls for participation in demonstrations planned without the consent of the government. It took effect on Feb. 1. (Editing by Tom Heneghan)