President Andrzej Duda took part in commemorations in Wieluń, the first Polish city to be bombed by the Germans at 4:40 am on September 1, 1939.
Meanwhile, Prime Minister Mateusz Morawiecki, along with officials including Defence Minister Mariusz Błaszczak, took part in ceremonies on the Westerplatte peninsula in the northern city of Gdańsk.
'We paid a terrible price'
Addressing those gathered in Wieluń, the Polish president said the city was "a symbol of cruelty" against civilians, "a symbol of breaking the rules, a symbol of that war" for the Polish people.
"This was an act of terror, not an ordinary war," he said. "From the very beginning, it was a violation of all rules of war."
Duda noted that 6 million Polish citizens lost their lives during World War II and that most of those killed were civilians, with military casualties totaling around 250,000.
"A small fraction of the 6 million Polish wartime deaths were those who actually fought with arms in their hands," he said.
"The rest were civilians, many of them brutally murdered, killed in bombings, tortured," he added.
"We paid a terrible price for that attack," he told those at the ceremony, adding that the Polish people were proud that they "never surrendered, never gave up."
Duda has previously said that his country was "the guardian of the memory" of World War II amid attempts by some to rewrite history and "blur the truth about the war."
He also told Poles that remembering the events decades ago was important because they were "a lesson in history that must not be forgotten."
The first bombs of World War II fell on a hospital in Wieluń, killing 32 people, including 26 patients. In all, the Germans killed around 1,200 in the city and surrounding areas that day, according to some estimates.
Wieluń, in central Poland, was located close to the Polish-German border before national boundaries shifted after the war.
Prime Minister Mateusz Morawiecki speaks at a commemorative event at the Westerplatte peninsula in the northern Polish city of Gdańsk. Photo: PAP/Adam Warżawa
The Gdańsk commemoration, meanwhile, traditionally started at 4:45 am, the time on September 1, 1939 that the German battleship Schleswig-Holstein began shelling a Polish military depot on Westerplatte in the first battle between Polish and German soldiers of WWII.
Sirens wailed and the Polish national anthem was played at a monument honouring those who defended the Polish coast.
The week-long defence by the Poles against overwhelmingly larger German forces became a symbol of the heroism of Polish soldiers.
Defence Minister Mariusz Błaszczak attends the commemorations at Westerplatte on Wednesday. Photo: PAP/Adam Warżawa
'Operation to destroy Polish nation'
During the ceremony, the Polish prime minister said that Westerplatte saw the start of the worst disaster of the 20th century and one of the bloodiest conflicts in the history of mankind.
He noted that exactly 82 years ago, on September 1, 1939, World War II broke out after Nazi Germany invaded Poland following the signing of a non-aggression pact with the Soviet Union.
Morawiecki told those at the ceremony that World War II was "an operation to destroy and completely annihilate the Polish nation."
"It was also an economic operation based on a gigantic robbery, seizure of property and the bank deposits of the Polish people, seizure of goods and theft of factories," he added. "It was a large-scale robbery and destruction that have never been compensated for."
Morawiecki also said that "the Germans and the Soviets deprived the Polish people of freedom, independence and opportunities for normal development for decades."
He added that Polish soldiers and civilians bravely fought on all fronts of World War II "for humanity, for independence, for a better world, for a free homeland, for a free Europe, for a world without war."
"It was the Poles who made the greatest sacrifice," he also said.
"It is the Germans who bear full and absolute responsibility for what happened then," he told those at the ceremony.
Source: IAR, PAP, TVP Info