Speaking to the Independent, Mr Landsbergis said all three Baltic states felt increasingly threatened by what they saw as a return to imperialistic thinking by their giant neighbour in the east.
Unless the West registered a strong protest now, he warned, Lithuania, Latvia and Estonia may prove unable to resist being sucked back into a Moscow-dominated orbit, ending up as little more than satellite states.
"If this trend towards the `reintegration' [of former Soviet republics] is not opposed strongly enough, Russia will see that as a signal it can proceed," he said. "It is time for a clear message from the West: 'Nyet!' - 'No, Never again'."
Like many in the Baltics, Mr Landsbergis was alarmed by last month's vote in the Russian Duma, denouncing the dissolution of the old Soviet Union in late 1991.
He has watched anxiously as the former Soviet republics of Russia, Belarus, Kazakhstan and Kyrgyzstan signed an agreement aimed at forging closer economic ties and, earlier this week, as Russia and Belarus forged a closer political and military union.
"I was saddened to see the people of Belarus giving up the fight for self-determination," said Mr Landsbergis, a former musician who leapt to fame as the leader of Lithuania's bloody independence struggle in 1990- 1991 and who, after a spell as President, now heads the opposition. "The danger for Lithuania has increased."
Although the drive to restore the Soviet Union is being spearheaded by Russian Communists, the cause has been taken up in part by President Boris Yeltsin, anxious to boost his chances in June's presidential election. It was President Yeltsin, after the agreement between Russia, Belarus, Kazakhstan and Kyrgyzstan, who said that "the new community is open to other states ... maybe the Baltic states or perhaps, for example, Bulgaria."
Baltic leaders stress that after 50 years of enforced incorporation into the Soviet Union they have no interest in joining another Moscow-led alliance. Instead, they plan to redouble their efforts to join the European Union and Nato.
"If some of the former Soviet republics want to form a new union, that is up to them, but it is not what we want," said an Estonian foreign ministry spokesman. "The more pressure we feel coming from Russia, the more we want to bind ourselves to the West."
Officially, Baltic politicians regard membership of the EU and Nato as two sides of the same coin. Unofficially, however, some recognise that, given the scale of Russian objections to Nato's enlargement and their geographical position, they are unlikely to join the Western military alliance in the near future, if at all.
While not withdrawing their attempts to join Nato, some Baltic leaders are exploring the possibility of an alternative form of security guarantee from the West. They are also beginning to place more emphasis on the more realistic goal of EU membership, against which Russia has not expressed such fierce opposition.
"Our main priority must be to turn our economies round so that, effectively, we actually do integrate with the West," said Mart Laar, the former Estonian prime minister who spearheaded what is widely seen as one of the most successful economic transformations in the former Communist bloc.
Of all the Baltic capitals, Tallinn, with its beautiful mediaeval heart, smart shops and boatloads of Finnish visitors, has moved the furthest away from the grey days of Soviet rule. To many people it looks as if it is already part of the West.
"We hope that by the time we join the EU it may well have a defence element," said Mr Laar, a member of the younger generation that is trying to take a less alarmist view of developments in Russia. "But in the end, it is foreign investment that really counts. If the West is prepared to put its money here, then that is a higher guarantee of our security than any number of tanks."Reuse content