Law enforcement officers searched an opposition group's office in Moscow, an activist said.

About 20 people who identified themselves as security and criminal police officers raided the office of the United Civil Front headed by former world chess champion turned fierce Kremlin critic Garry Kasparov, his aide, Marina Litvinovich, told The Associated Press.

Litvinovich said the officers made copies of all documents they found of interest and she was told the search was connected to suspicions that Kasparov's group was involved in extremist activity.

Law enforcement officials were not immediately available for comment.

Kasparov said the search was an example of Russian authorities using a recently passed law on fighting extremism to crack down on dissent.

"That's an important message not only for people in Russia, but also for the rest of the world to see how new laws, which are supposed to fight extremism, are used against political opposition," he told AP Television News.

Kasparov added that he saw his fight for more political freedoms in Russia as a key personal mission.

"I was defending Russia's colors for 20 years, playing chess for the Soviet Union and Russia. And I'm defending its colors now," he said.

Litvinovich said she believed the search was meant to intimidate the activists ahead of an opposition march planned for Saturday.

She said the action was expected to draw several thousand liberals protesting what they call Russia's democratic backsliding. City authorities have banned the march, but have allowed the activists to hold a rally in one city square, she said.