Andrzej Duda was joined in Warsaw on Sunday by the representatives of dozens of other countries, including U.S. Vice President Mike Pence and the two top leaders of Germany, Chancellor Angela Merkel and President Frank-Walter Steinmeier.
Polish soldiers sang the national anthem and raised the flag at Pilsudski Square, a vast space in the center of the city that was almost totally razed to the ground by the German forces during the war, which began on Sept. 1, 1939.
The ceremonies in Warsaw follow commemorations in the early hours of Sunday at the exact time of attacks on Poland, in Wielun and on the Westerplatte Peninsula in Gdansk.
Poland's prime minister spoke of the need for redress during observances of the start of World War II as he talked about the losses that Poland suffered during almost six years of Nazi German occupation.
Mateusz Morawiecki said during ceremonies at the Westerplatte Peninsula on the Baltic coast, where the war's first battle was fought that the war "meant not only fire for the Polish homes, it meant the death of Poland's hopes, Poland's future, the end of Poland's science, Polish universities, Polish factories."
He said that "For this reason we should talk about these losses, we should .... demand redress."
Poland's nationalist government has been raising the issue of reparations from Germany ever since it took power in 2015. Germany says that matter is closed.
The presidents of Germany and Poland have opened daylong observances of the 80th anniversary of World War II start with a ceremony in a central Polish town that was the first target of Nazi Germany's deadly bombings.
The ceremony in Wielun, attended by German President Frank-Walter Steinmeier and his Polish counterpart Andrzej Duda, started Sunday at 4.40 a.m., the exact hour that, according to survivors, the war's first bombs fell, killing civilians.
Minutes later, observances began at Westerplatte Peninsula, on the Baltic coast, the site of the war's first battle as Polish troops put up resistance.
U.S. Vice President Mike Pence, German Chancellor Angela Merkel and many other leaders will also attend the main event in Warsaw.
President Donald Trump canceled his arrival to deal with Hurricane Dorian approaching Florida.