Deputy Prime Minister Krzysztof Gawkowski led observances in Wieluń, the first Polish city bombed by the Germans at 4:40 a.m. on September 1, 1939.
Meanwhile, President Karol Nawrocki, Prime Minister Donald Tusk and Defence Minister Władysław Kosiniak-Kamysz took part in ceremonies on the Westerplatte peninsula in the northern city of Gdańsk.
'Merciless power of fascist evil'
Addressing those gathered in Wieluń, Gawkowski said that the Polish town witnessed "the terrible, merciless power of fascist and German evil" that "put an end to carefree life and dreams of peace."
The first bombs of World War II fell on a hospital in Wieluń, killing 32 people, including 26 patients. Historians estimate around 1,200 residents were killed in the city that day.
Parliamentary Speaker Szymon Hołownia, in a letter read at the ceremony, wrote that "war begins not with gunfire, but with contempt, greed, radicalism and acceptance of any form of violence."
He added: "Today the world is being shaken by armed conflicts, with cities bombed, civilians killed, international law and fundamental human rights violated, and hatred and animosity constantly fueled.
"Therefore, from Wieluń, a symbol of innocence destroyed by armed madness, the call for peace must resound louder than ever."
Ensuring peace requires investment in armed forces: president
In Gdańsk, the commemoration traditionally began at 4:45 a.m., when the German battleship Schleswig-Holstein opened fire on a Polish military depot at Westerplatte, marking the first battle between Polish and German soldiers in the war.
Sirens wailed and the Polish national anthem was played at a monument honouring those who defended the Polish coast.
Nawrocki said the weeklong defence of Westerplatte against vastly superior German forces became a symbol of Polish heroism and patriotism.
"Ensuring peace today requires investment in the Polish armed forces," he said.
Tusk called Westerplatte "a sacred place where we gather every year, which forces us to remember not only the tragedy of World War II and its millions of victims, but also that Poland can never again fall victim to aggression."
Nearly 6 million Polish citizens, including 3 million Polish Jews, were killed in World War II.
Officials said the country emerged from the conflict devastated, looted and with its economy in ruins.
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Source: IAR, PAP