The ceremony at the monument on the Westerplatte peninsula will also be attended by presidents Francois Hollande of France, Joachim Gauck of Germany, Dalia Grybauskaite of Lithuania, Andris Berzins of Latvia, Toomas Hendrik Ilves of Estonia and many other heads of state.
The peninsula was the site of the first battle of WWII in Europe, where the German warship "Schleswig-Holstein" opened fire on the Polish army base on 1 September 1939. A 25-metre monument of 236 granite blocks was erected there in tribute to the Polish defenders.
In the European Centre of the Solidarity Movement in Gdansk, the Croatian president will attend a conference entitled "The Legacy of World War II after 70 Years" and a panel on European integration as a lesson from World War II.
Ceremonies will be held throughout the day to commemorate over 50 million people killed worldwide during WWII, including six million Poles.