The German president, Frank-Walter Steinmeier, apologized to Poland for the Nazi "tyranny", 80 years after the start of World War II.
Steinmeier and other world leaders are in Poland to commemorate the beginning of the conflict.
This Sunday morning, a ceremony was held in the Polish city of Wielun, where the first German bombs fell.
Steinmeier condemned the "desire to annihilate" that led to the attack.
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"I bow my head to the Polish victims of German tyranny and I apologize," Steinmeier said in German and in Polish.
The president spoke with his Polish counterpart, Andrzej Duda, who considered the attack by Nazi Germany against Poland as "an act of barbarism."
At the ceremony, which began shortly after four in the morning, a minute of silence was kept in memory of the victims.
Wielun had to give an idea of the kind of war that was going to be. It was going to be a total war, a war without rules, a destructive war, "Duda said.
Other world leaders, including German Chancellor Angela Merkel and U.S. Vice President Mike Pence, will meet later in another ceremony to be held in the Polish capital, Warsaw.
Poland was one of the countries that suffered most from the ravages of World War II: some 6 million of its citizens were killed.
Eighty years later, the European nation continues to demand compensation from Germany for the death and destruction inflicted.
Recent requests for redress by Polish Prime Minister Mateusz Morawiecki, whose ruling Law and Justice (PiS) party has been accused of fueling nationalist sentiments, have fueled tensions with Germany.
A Polish parliamentary committee is still calculating the amount of compensation, but Germany argues that this is an issue already resolved.
How did the Second World War begin?
At dawn on September 1, 1939, the German Luftwaffe (air force) bombed the city of Wielun, a town that lacked military relevance.
It is estimated that thousands of people died in the bombings, designed to sow terror among the civilian po[CENSORED]tion.
After the attack ordered by Nazi leader Adolf Hitler, Britain gave Germany an ultimatum to cease military operations.
When this ultimatum was ignored, Britain and France declared war on Germany on September 3, thus beginning a 6-year conflict that would kill millions of people.
How is the anniversary commemorated?
In addition to the ceremony in Wielun, memorial events are also scheduled in Warsaw and at the former Westerplatte military post.
In Westerplatte, where German Nazi battleships attacked a Polish military base on September 1, 1939, Morawiecki and the Vice President of the European Commission, Frans Timmermans, attended a commemorative event at dawn.
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Also this Sunday, Steinmeier, Duda and Pence will deliver speeches at the ceremony at the Pilsudski Square in Warsaw, where the Tomb of the Unknown Soldier is located.
Major world leaders such as the president of the United States, Donald Trump, who is busy dealing with preparations for Hurricane Dorian, Frenchman Emmanuel Macron and British Prime Minister Boris Johnson will not be present.
Around 40 foreign delegations will attend the commemorations, according to the Polish presidency.