The warning was made in a telephone call to Lithuania's party leader, Algidas K. Brazauskas, and then passed on to local leaders of Sajudis, the Lithuanian independence movement, the activist said.
So far, the Soviet leadership has sought to influence citizens of the various republics with words alone. But in recent days, the words have become tougher, and some analysts say the Kremlin may be laying the groundwork for direct intervention--possibly the arrest of nationalist leaders or a shake-up of local party organizations.
Gorbachev spoke by telephone Sunday to Brazauskas, a Brazauskas aide said, declining to elaborate.
A spokesman for Sajudis, Romaldus Ozolus, said Brazauskas met Monday with Sajudis leaders and told them that "Gorbachev is concerned about what's happening in Lithuania--that what had happened so far was worth supporting but that now we'd gone too far."
Previously, the Soviet party had taken a conciliatory stance as the republics felt their way under Gorbachev's policies of openness and reform. For example, the Kremlin stood by quietly as language laws similar to the proposed Moldavian measure were enacted earlier this year in Lithuania, Latvia and Estonia.
Until recently, the Kremlin generally refrained from criticizing the popular front movements formed in many republics to give ethnic minorities a political voice. The aim was to persuade leaders in the republics that they could achieve their goals without seceding from the Soviet Union, a step that is permitted, in theory, by the Soviet constitution.
But now, clearly feeling threatened by growing cries for independence, Moscow has decided to speak out decisively against the nationalist moves.
"Moscow has changed its tactics and seems to be preparing the way for harsher actions against nationalists," one Western analyst said, forecasting a possible shake-up of Communist Party bosses in some of the more active republics.
But the analyst said the Soviet leadership will probably delay any major action until after the Supreme Soviet, the national Parliament, convenes next month.
"To act without some kind of a mandate would seriously undermine Gorbachev's reform program internally," he said.
In Kennebunkport, Me., where he is on vacation, President Bush appeared to send a signal to the Soviets advocating restraint.
Asked if he feared that recent hard-line statements could presage a crackdown against opposition groups in the Baltics, Bush declared: "I hope not." In a brief question-and-answer session with reporters, he went on to praise the restraint and "understanding" that Gorbachev had shown in his response to developments this month in Poland, where the first non-Communist government in the East Bloc since World War II was formed.
Meanwhile, the change in Moscow's mood was reflected Monday in an article in Pravda, the official organ of the party, which lashed out at supporters of the language law in Moldavia and warned that if deputies there pass such a law, their votes would signal support for "ethnic strife, the isolation of Moldavia and disruption of its ties with other republics."
Soviet television accused leaders of the independent popular front movement in Moldavia of whipping up "nationalist hysteria."