As world leaders gathered in Poland on the weekend to commemorate the start of World War II, German President Frank-Walter Steinmeier asked his hosts to forgive his country's past "tyranny".

Steinmeier was there to mark the moment 80 years ago when German bombs fell, ahead of its tanks rolling across the border into its neighbor to the east, triggering six years of global war and suffering.

Steinmeier lamented Germany's "desire to annihilate" in the middle years of the 20th century, saying: "I bow my head before the Polish victims of Germany's tyranny and I ask forgiveness."

He made the comments in a speech, given in both German and Polish, in the Polish city of Wielun, which was the first place to be on the receiving end of German bombs during the conflict.

His Polish counterpart, Andrzej Duda, said Germany's attack on his country that claimed 6 million Polish lives, including those of 3 million Jewish civilians, was "an act of barbarity".

"Wielun was to show what kind of war it would be, that it would be a total war, a war without rules, a destructive war," the BBC quoted Duda as saying.

The dawn bombing raid prompted the United Kingdom to warn Germany to cease military operations or face the consequences and, on Sept 3, 1939, after Germany ignored the warnings and continued to attack Poland, the UK and France declared war on the Nazis.

Other events took place elsewhere on Sunday to commemorate the beginning of the world's most deadly conflict, including one in the Polish capital Warsaw, which was attended by German Chancellor Angela Merkel and US Vice-President Mike Pence as well as French Prime Minister Edouard Philippe, Belgian Prime Minister Charles Michel, and the leaders of Croatia, the Czech Republic, Hungary, Lithuania, and Slovakia. DW News, the English-language branch of German international broadcaster Deutsche Welle, noted that United States President Donald Trump intended to attend but was unable to do so after his attention was diverted toward a hurricane approaching his nation.