President Andrzej Duda marked the 82nd anniversary of the outbreak of World War Two in Wielun, a town in central Poland levelled by German bombers in the first hours of the war on September 1, 1939.
The ceremony started before 4.40 a.m., the time when German aircraft began bombing the town despite its being of no military significance.
The president said that the local hospital, which was marked with a big red cross, was one of the first targets for German pilots.
"It was an act of terror, it was not an ordinary war," Andrzej Duda said.
"From the very beginning, it was about breaking all the rules of warfare that were binding then as they are now," he added.
Andrzej Duda also said Poland lost six million citizens during World War Two, including Poles, Jews, Armenians and other nationalities living in the country at that time.
"It was close to 17 percent of Poland's population and no other country involved in World War Two suffered such losses," Andrzej Duda added.
Before the war, Wielun had around 16,000 residents and was located only 21 kilometres from the German border. On the morning of September 1, 1939, Nazi Germany dropped 380 bombs on the city, killing between 1,200 and 2,000 residents. (PAP)