MOSCOW, AUG. 23 -- Thousands of people poured into the streets of the three capital cities in the Baltic region today to protest the 1939 Stalin-Hitler pact that permitted Soviet takeover of the region.

The nonaggression pact between Kremlin leader Joseph Stalin and German fuehrer Adolf Hitler, signed 48 years ago today, allowed incorporation of Latvia, Lithuania and Estonia, stretching along the Baltic Sea coast, into the Soviet Union. The treaty also delayed the Soviet entry into World War II against Germany.

In a period that has seen an unusual number of disputes between local nationalities and Soviet authorities, today's protests focused attention on the region most likely to produce protests, western experts on Soviet nationalities said.

A similar demonstration was held in June in Riga, Latvia, where thousands protested the deportation of Latvians to labor camps by Stalin in 1941.

Demonstrations by other nationalities against Soviet authorities occurred last December in Alma Ata, Kazakhstan, and in Moscow last month. The latter was a clash between police and Crimean Tatars.

Today's coordinated demonstrations were described by western diplomats here as the biggest organized protests against Soviet rule in the Baltic region in memory. They were seen by these envoys as a result of glasnost, or openness, the policy of Communist Party General Secretary Mikhail Gorbachev.

In demonstrations synchronized at noon today, protesters in the capital cities of all three republics defied police barricades to show their disdain for continued Soviet rule.

In Riga, Latvia, about 2,000 demonstrators met at the monument to war victims in the city center, according to two Latvian sources reached by telephone tonight. Others crowded into the large park area surrounding the monument, deterred from approaching the statue by 500 armed police, the sources said.

{Residents reached in Riga by telephone, reported that television there carried film of the demonstration, including interviews with protesters complaining of police brutality, United Press International reported, quoting an exile in Sweden.

{The demonstration was organized by the Helsinki '86 group and resulted in eight arrests, according to the World Federation of Free Latvians, in Rockville.}

In Vilnius, Lithuania, more than 500 demonstrators gathered at St. Anne's Church at the old town center, the Reuter news agency reported from the scene.

Standing by a statue in front of the church, a local woman demanded independence for the three republics. Reuter added that other demonstrators wore black armbands, sang Lithuanian patriotic songs and joined in chants of "Freedom, Freedom."

In Tallinn, Estonia, hundreds gathered at the statue of an Estonian national hero at Hirve Park in the city center, the Soviet news agency Tass reported, and spoke about "the national feelings of Estonians." The protesters laid flowers at a statue honoring the widow of Estonian hero Epos Kalevipoeg.

Plainclothesmen watched the demonstrations in Vilnius and Tallinn, sources said.

The protests also documented the tensions between Soviet officials and local residents that have lingered in the region for nearly five decades.

By reputation, the Baltic peoples maintain the most vehement feelings for their independence of all Soviet nationalities.

Articles in the Soviet press warned against the protests in a media campaign that began two weeks ago and climaxed this weekend.

The articles accused the western media of inspiring the demonstrations. "Foreign 'radio voices' began to prepare for the gathering long ago," Tass said in the dispatch from Tallinn today.

The nonaggression pact was signed on Aug. 23, 1939, by Soviet Foreign Minister V.M. Molotov and Nazi Foreign Minister Joachim von Ribbentrop.

The following month, after the Nazi invasion of Poland that began World War II, Soviet forces occupied eastern Poland and the three Baltic republics, which were annexed by Moscow on Aug. 3, 1940.

Soviet historian Herman Trukan, in an interview with Tass released on the eve of the demonstrations, gave the official Soviet version of these events.

The Baltic peoples voted to join the Soviet Union and Soviet troops came into the region to protect them against the Nazis, he said.

All three republics were independent from 1918 to 1939 and many western countries, including the United States, do not recognize their incorporation into the Soviet Union.

Twenty U.S. senators wrote to Soviet officials asking that the demonstrations be allowed to take place.

Pravda, the official daily of the Communist Party, said today, "The senators' message coincides . . . with the overtly instigatory anti-Soviet campaign launched in recent weeks by western radio voices broadcasting to the Soviet Union, including those subversive radio stations which are financed and directed by the U.S. Central Intelligence Agency."

The reference was apparently to Radio Liberty and Radio Free Europe, which are financed by the U.S. government.