Polish President Andrzej Duda laid a wreath at the Monument to the Fallen and Murdered in the East in Warsaw on Saturday morning.
A mass, attended by Defence Minister Antoni Macierewicz, was held later in the courtyard of the Katyń Musuem in the Polish capital.
"As a result of September 17, Poland suffered for decades, with two great powers pursuing to destroy the Polish nation," Macierewicz said in his speech.
Another ceremony to mark the 77th anniversary of the Soviet invasion of Poland was held at the Monument to the Fallen and Murdered in the East on late Saturday afternoon.
Antoni Macierewicz and Warsaw mayor Hanna Gronkiewicz-Waltz took part in the event, along with veterans and scouts.
Basic facts
At 3.30 am on 17 September 1939, the Polish ambassador in Moscow was handed a note, in which Moscow announced that the Polish state had ceased to exist.
Initially, the Soviet Union sent in 450,000 soldiers, though by the end of September this figure had risen to one and a half million.
Poland was then caught between German Nazi forces advancing from the west and Stalin's forces from the east.
On 28 September, Germany and the Soviet Union drew up boundaries under the Molotov-Ribbentrop Pact signed a month before, which agreed spheres of influence within Poland, effectively snuffing out the independence the country won in 1918.
Following the Soviet invasion, around 15,000 Polish officers were deported to camps in Kozelsk, Ostashkov and Starobelsk.
Under Stalin's decree of March 1940 the officers were executed by the NKVD security forces, and their bodies were buried in mass graves in Katyn, Kharkov and Mednoye.
Mass deportations followed to Siberia, which included, according to various estimates, from 550,000 to nearly one and a half million Poles.
"The dramatic course of those events resulted in a situation where Poland had no freedom and independence for many decades," historian Jan Żaryn told Polish Radio. (ał)
Source: IAR, PAP