Officials attended ceremonies at sites including Warsaw's Monument to the Fallen and Murdered in the East.
President Andrzej Duda took part in a commemorative event in the eastern city of Białystok, where he paid tribute to soldiers who fought to defend Poland against the invading Red Army in September 1939.
He also opened a new museum commemorating Poles deported to Russia's remote Siberia region decades ago.
As they marked the anniversary, officials said September 17, 1939 was one of the most tragic dates in recent Polish history.
Invaded by two military powers
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.
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.
Source: PAP, IAR, ipn.gov.pl