Top government officials and political leaders are expected to attend ceremonies on Sunday at sites including Warsaw's Monument to the Fallen and Murdered in the East.
In the days leading up to the anniversary, officials said that September 17, 1939 was one of the most tragic dates in recent Polish history.
At dawn on September 17, 1939, Soviet troops invaded Poland following a secret agreement with the German Third Reich.
Poland was then caught between German Nazi forces advancing from the west and Soviet forces from the east.
Following the invasion, some 250,000 Polish soldiers were captured by the Soviets, who later executed thousands of prisoners of war.
Mass deportations of the civilian population followed, with up to 1.5 million Poles transported away into the Soviet interior, mainly to Siberia and Kazakhstan, according to some estimates.
Tens of thousands of ethnic Poles killed in the former USSR before the 1939 Soviet invasion were remembered at memorial ceremonies in Warsaw last month.
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Source: IAR, PAP, polskieradio.pl