Key sponsors include Republican U.S. Senator John McCain, Democratic U.S. Senator Joseph Biden, former Czech President Vaclav Havel, and former Swedish Prime Minister Carl Bildt.
Landsbergis told RFE/RL that the letter is meant as a wake-up call to leaders in the West who believe Russia is developing as a democracy: "It is warning to Western democracies, societies, and communities -- and also for some leaderships -- which are a little bit too naive, and they believe that Russia is still a democracy, [that it] is making progress and reforms."
Landsbergis said the letter is also intended to speak to the dwindling corps of democracy advocates in Russia:
"And also, it is a signal of support for those few in Russia itself who do believe still in democracy and are fighting for democracy. It's a signal that, 'You are not alone. You are not forgotten,'" he said.
A U.S. congressional staff member who asked not to be identified told RFE/RL that the idea for the letter came from a U.S. think tank allied with the opposition Democratic Party.
Landsbergis said the original source for the letter is insignificant.
"It's more interesting how this letter will work," Landsbergis said. "It is an initiative of real and active democrats, democrats not only because of belonging to the party but [also] democrats from belief."
He said he hopes the letter will cause Western leaders to assess Vladimir Putin's government more realistically -- that is, as an increasingly authoritarian regime.
In Moscow, Sergei Markov, director of the Moscow Center for Political Studies, said the signers of what is being called the "Letter of 100" misunderstand Putin's governance. He said the Russian president is uninterested in political theories but only in practical results.
"Putin thinks that political institutions can be useful for Russia only if they work," Markov said. "And sometimes, because Russia has a weak civil society and weak government institutions, not all democratic institutions work. So, Vladimir Putin prefers to use democratic institutions when they work and use other institutions when democratic institutions don't work."
Markov said it is an error to consider Russia as a country in transition from communism. He said that idea became outmoded soon after the 1991 breakup of the Soviet Union: "Call it 'managed democracy,' meaning that Russia is not a totalitarian country, not a democratic country, but Russia is on the way to democracy. On the way from what? From communist dictatorship? No! Russia is not on the way from communist dictatorship, because the Russian communist dictatorship already fell down into chaos and anarchy. Now, Vladimir Putin is trying to move Russia from chaos and anarchy toward a more stable democracy through this transition period."
The "Letter of 100" opens with expressions of sympathy and condolence for what it calls the "heinous act of terrorism" in the North Ossetian town of Beslan in early September, in which roughly 350 people were killed, many of them children. But it charges that Russia's leaders are using the tragedy as an excuse to suppress democracy still further. Among other measures, Putin has proposed doing away with elections by popular vote for governors in Russia's 89 regions. Candidates would instead be selected by the president and approved by regional assemblies.
Former Czech President Havel -- himself a famous dissident before the fall of communism in his country -- said yesterday that the letter is not an act of hostility toward Russia.
"I have always expressed the idea that friends should speak to each other openly and should tell each other what they think," Havel said.
The letter says the time has come for Western governments to take sides clearly with democratic forces in Russia and "to speak the truth about what is happening in Russia."