Few places saw death and destruction on the scale of Poland. It lost about a fifth of its population, including the vast majority of its 3 million Jewish citizens.
After the war, its shattered capital of Warsaw had to rise again from ruins and Poland remained under Soviet domination until 1989.
Ceremonies began at 4.30 a.m. (0230 GMT) in the small town of Wielun, site of one of the first bombings of the war on Sept. 1, 1939, with speeches by Polish President Andrzej Duda and his German counterpart, Frank-Walter Steinmeier.
Parallel events, attended by Prime Minister Mateusz Morawiecki and European Commission deputy chief Frans Timmermans, were held in the coastal city of Gdansk, site of one of the first battles of the war.
Morawiecki spoke of the huge material, spiritual, economic and financial losses Poland suffered in the war.
"We need to talk about those losses, we need to remember, we need to demand truth and demand compensation."
For Poland's Law and Justice (PiS) party, the memory of the war is a central plank of its "historical politics", aiming to counteract what it calls the West's lack of appreciation for the extent of the nation's suffering and bravery under Nazi German occupation.
PiS politicians have also repeatedly called for war reparations from Germany, one of Poland's biggest trade partners and a fellow member of the European Union and NATO. Berlin says all financial claims linked to World War Two have been settled.
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.
Articles paid for by a foundation funded by state companies, showing Poland's experience in the war, appeared in major newspapers across Europe and the United States over the weekend.
As part of its campaign, the Polish National Foundation also paid for supplements in some newspapers consisting of a copy of their front pages from Sept. 2, 1939, that highlighted the Nazi German army's attack on Poland.
Wartime remembrance has also become a campaign theme ahead of a national election due on Oct. 13, with PiS accusing the opposition of failing to protect Poland's image.
"Often, we are faced with substantial ignorance when it comes to historical policy ... or simply ill will," Jaroslaw Sellin, deputy culture minister, told Reuters.
Merkel and Pence, who is coming after President Donald Trump abruptly cancelled a planned trip due to a hurricane, will participate in events later in the day in Warsaw.
The cancellation of Trump's visit is a disappointment to the PiS government, which is seen as one of Washington's closest allies in Europe. Polish and U.S. officials have said another visit could be scheduled in the near future.
Opinion polls show PiS is likely to win the October ballot. The party's ambition is to galvanise voters and disprove critics by winning a majority that would allow it to change the constitution.