In his first state-of-the-nation address on Wednesday Russian President Dmitry Medvedev proposed amending the Constitution to extend the presidential and parliamentary terms to six and five years, respectively.
The movement, tentatively called Solidarity, is currently being formed by Kasparov, leader of the United Civil Front, Nikita Belykh, a founder and the former leader of the Union of Right Forces (SPS) party, the leaders of the Yabloko opposition party's St. Petersburg and youth branches, Maxim Reznik and Ilya Yashin, as well as politicians Vladimir Milov and Boris Nemtsov.
"Solidarity must become one of the few forces that will protect the Russian Constitution," Nemtsov said.
On behalf of the other founders he described Medvedev's move to extend the presidential term as an "extremely dangerous, harmful and anti-Russian initiative."
The new party has set December 13 as the date of its founding congress.
Nemtsov called the date "symbolic" as Russia marks Constitution Day on December 12.
The Vedomosti business daily said earlier on Thursday that, according to a Kremlin source, Medvedev needed to amend the Constitution to secure a longer term in office to carry out unpopular social reforms and to allow his predecessor, current Prime Minister Vladimir Putin, to return as president.