In his speech, Polish President Duda thanked Steinmeier for his presence at the painful anniversary.
"Eighty years ago, at this very moment, all hell rained down on Wielun — fuelled by German racist barbarity and the desire to annihilate," said Steinmeier. The German president also asked for Polish forgiveness. "I bow before the Polish victims of German tyranny, and I ask for your forgiveness."
Minutes later, Polish Prime Minister Mateusz Morawiecki and the deputy head of the European Commission Frans Timmermans led observances at Westerplatte Peninsula, on the Baltic coast, the site of the war's first battle as Polish troops put up resistance.
The main event in Warsaw will have a long list of participants, including German Chancellor Angela Merkel, French Prime Minister Edouard Philippe, Belgian Prime Minister and President-elect of the European Council Charles Michel as well as leaders from Croatia, the Czech Republic, Lithuania, Slovakia, Ukraine and Hungary.
US President Donald Trump was supposed to attend, but he canceled his planned trip saying that he needed to deal with a hurricane approaching the southeastern United States. Vice President Mike Pence is participating in the event in Trump's place.
Few places suffered the level of death and destruction seen in Poland during World War II. It lost about a fifth of its population, including the vast majority of its 3 million Jewish citizens. The country remained under German occupation for more than five years.
After the war, Poland had to rise again from ruins and it remained under Soviet domination until 1989.
For Poland's ruling Law and Justice (PiS) party, the memory of World War II is a central plank of its "historical politics," aiming to counteract what it says is a lack of appreciation in the West of the extent of the nation's suffering and bravery under Nazi German occupation.
Critics say the party's ambition is to fan nationalism among voters at a time when populists around the world are tapping into historical revisionism. PiS says the country's standing on the global stage and national security are at stake.
In a move likely to increase tensions between Poland and Germany, fuelled under the PiS government, Polish PM Morawiecki renewed calls for wartime reparations in recent days.
Germany, one of Poland's biggest trade partners and a fellow member of the European Union and NATO, has repeated that all financial claims linked to World War II have been settled.