The group, led by chess champion Gary Kasparov, a vociferous critic of Putin and his government, concedes they can't stop the popular president from being re-elected in March but vow to nip any effort to extend his term, the BBC reported.
"We concede that we are a minority, but a minority united in its belief that democratic values are under threat in Russia," the group said in a statement Tuesday.
Russia's two main liberal parties suffered a crushing defeat in December's parliamentary elections and are now barely represented, but United Russia, backed by Putin, won more than 300 seats in the 450-seat Duma.
"Those who represented democracy, the Union of Right Forces and Yabloko, have been consigned to the dustbins of history," Kasparov told the Russian daily Nezavisimaya Gazeta.
"We have to start all over again."
Kasparov's group, "2008: Free Choice," issued a statement accusing Putin of trying to consolidate his "czar-like" power at all costs and criticizing his control over parliament and the media.