MOSCOW, NOV. 5 -- Alexander Dubcek, the former leader of Czechoslovakia who spearheaded the "Prague Spring" reforms that were crushed by a Soviet-led invasion in 1968, has sent a message of congratulations to the Kremlin leadership on the 70th anniversary of the Bolshevik Revolution, a leading Soviet official said here today.

The news of Dubcek's letter was announced by Central Committee official Georgi Shaknazarov at a press conference one day after a leading Soviet historian suggested that a new assessment of the Prague Spring -- and its suppression by Warsaw Pact troops -- may now be in order.

The Prague Spring was a brief, popular experiment with democratic socialism that introduced greater economic and cultural freedoms and more open political debate in Czechoslovakia. Soviet leader Mikhail Gorbachev is now promoting in the Soviet Union many of the reforms that ultimately led to Dubcek's ouster.

Dubcek, a 65-year-old pensioner who lost his membership in the Czech Communist Party after he was dumped from power, also welcomed in his message Moscow's efforts to build democratic socialism, according to Czech emigre sources in the West.

In addition to Dubcek, 20 former Czechoslovak officials who were purged after the invasion reportedly wrote to an international meeting of communist and leftist parties gathered for the anniversary in Moscow.

In 1968, the Kremlin ordered the invasion of Czechoslovakia out of fears that the country's experiments were a threat to socialist rule and Soviet hegemony. A review of that crackdown could send an important signal to Soviet allies in Eastern Europe, western diplomats said here today.

It also could destabilize the grip of aging leaders now ruling Czechoslovakia who gained power in the wake of the Soviet intervention, the diplomats added.

Czechoslovak leader Gustav Husak and a delegation of party officials flew home to Prague from Moscow this afternoon. They left before the Red Square parade, the highlight of anniversary festivities, which is scheduled for Saturday.

Husak, who assumed power after Dubcek was ousted in April 1969, left one day after Soviet historian Georgi Smirnov told a news conference, "I think we should think over the events of 1968," a reference to the period of reform followed by the invasion in Czechoslovakia.A Czech embassy official said that Husak and others left to attend a party meeting in Prague on Friday.

Despite Smirnov's suggestion, a leading Soviet official said today that the Prague Spring had encouraged both the East and the West to work toward detente.

Asked in a news conference today about the chances of an official Prague Spring review, Novosti news service director Valentin Falin said, "I think the links between the events of 1968 and the conclusions that were drawn on both sides, in the West and in the East, on the need to move from military confrontation to detente are not coincidental."

"The events were directly linked," Falin added, "for then probably in the West as well they realized that it was not possible to continue behaving in the old way that had seemed possible before what had happened in Czechoslovakia."