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, public broadcaster Polish Radio’s IAR news agency has reported.
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.
President Duda on Tuesday morning laid a wreath in front of a plaque in Warsaw commemorating Polish underground resistance fighters murdered by the Soviet secret police NKVD during the last two years of World War II.
Złożenie przez Prezydenta RP wieńca przy budynku dawnego Sowieckiego Trybunału Wojennego, prokuratury i NKWD https://t.co/hcxGDf8cMh
Later in the day officials were due to attend ceremonies at sites including Warsaw's Monument to the Fallen and Murdered in the East and the city's central Piłsudski Square, where a special Independence Concert was scheduled to be held in the evening.
On the eve of the anniversary, Defence Minister Mariusz Błaszczak said in a Twitter post that September 17, 1939 was one of the most tragic dates in recent Polish history.
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