Parliamentary Speaker Włodzimierz Czarzasty 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.
Addressing those gathered in Wieluń, Czarzasty said the Polish town "became the first target of a war that spread across Europe and the world."
German bombs fell on homes, streets and a hospital, killing hundreds of civilians, he said.
The first bombs to fall on Wieluń struck a hospital, killing 32 people, including 26 patients. Historians estimate that about 1,200 residents were killed in the city that day.
In a letter read at the ceremony, Nawrocki said Wieluń had become the first symbol in Poland of Nazi German brutality and the mass murder of civilians.
He added that historians were still investigating details of the German air raid on Wieluń, including its precise timing, the size of the forces involved and the course of subsequent attacks.
'Germany recognises its historical responsibility': ambassador
Miguel Berger, Germany's ambassador to Poland, who attended the ceremony, acknowledged that the attack on Wieluń was the first war crime committed by the German military during World War II.
He said the commemoration sent a message of peace, reconciliation and respect for human life to the world.
"However, it is important to keep the memory alive so that the victims, their relatives, friends and families are not forgotten. It is our duty to ensure that crimes such as those committed during World War II can never be repeated," he said.
Berger said Polish victims of World War II would be commemorated in Berlin and that "the memory of Wieluń will have a permanent place" there.
"Germany recognises its historical responsibility and feels committed to preserving the memory of the crimes committed by the Germans, without any 'ifs,'" Berger said.
Poland and Germany are now allies: deputy defence minister
Polish Deputy Defence Minister Cezary Tomczyk noted that Germany is now one of Poland's largest partners in the European Union and NATO.
He said the joint construction by Polish and German troops of the East Shield fortification system along Poland's eastern border with Russia and Belarus had symbolic significance in light of their shared history.
"This is a symbol of true reconciliation, a breakthrough, and the fact that we are in a completely different place today. We are a strong country, with one of the largest armies in NATO," Tomczyk said.
Polish president calls for 'truth and responsibility,' WWII reparations
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 had become a symbol of Polish heroism and patriotism.
He told the gathering that "remembering September 1 demands truth and responsibility."
He appealed to "the German chancellor and the German people" to "resolve the issue of reparations and compensation for World War II with both historical justice and the future in mind."
He also called on the German government to commemorate Polish victims of World War II in Berlin.
"We cannot remain indifferent to the fact that we still do not have a monument dedicated to the Polish victims of World War II in Berlin," he said.
'We must draw lessons' from WWII: PM
Tusk said the lessons of World War II "must influence the decisions and attitudes of the leaders of the free world" today.
"We must draw lessons from the horrors of World War II and focus on building solidarity and unity, both at home and across the free world," he said, adding that the NATO alliance "cannot be an alliance on paper."
"We must not allow ... the brown wave of aggression and contempt to ever repeat itself," he told the gathering, referring to the Nazi Brownshirts, Adolf Hitler's paramilitary street force.
Nearly 6 million Polish citizens, including about 3 million Polish Jews, were killed during World War II.
Polish officials said the country emerged from the conflict devastated, looted and with its economy in ruins.
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Source: IAR/PAP
Click on the audio player above for a report by Piotr Urbaniak