At 6 am - two Soviet fronts invaded the territories of the Second Polish Republic.

82 YEARS SINCE THE FOURTH PARTITION OF POLAND September 17, 1939 at 2 am Joseph Stalin, dictator of the Soviet Union met with the German Ambassador Friedrich Schulenburg to read to him and consult the final version of the diplomatic note which was to be delivered later that morning to Polish Ambassador to USSR - Waclaw Grzybowski. Its content basically stated that due to Poland’s loss of all its industrial capacity and cultural centers at the hands of the Germans, Warsaw ceased to exist as Poland’s capital. At 6 am - two Soviet fronts invaded the territories of the Second Polish Republic. The Red Army met fierce resistance from the Polish Border Control Corps units - which were not able to stop them, however, they did delay this aggression substantially.
On the following day, Red army units captured Lida, Trembowla, Nowogródek.
Bolsheviks would regularly capture Polish officers and KOP soldiers and execute them on the spot. In many Polish towns and villages, civilians, considered by the Russians to be Bourgeoisie (capitalism oriented), were murdered. Polish President Ignacy Moscicki was forced to evacuate into neighboring Romania which declared neutrality during the Polish-German war. Meanwhile Polish armed forces units intensively defended Polish cities and towns.
The Polish Spirit of battle, the Polish passion to fight for a Free Poland is well exemplified by the fact that underground military organizations began forming immediately.

The photo from PMA Library collection: book "Po jednej i drugiej stronie linii Ribbentrop - Mołotow. Okupacja niemiecka i sowiecka ziem polskich w latach 1939-1941 w porównaniu. T.1 Warszawa 2020 Instytut Pamięci Narodowej