Angela Merkel, Germany's chancellor, along with other European prime ministers including Russia's Vladimir Putin, France's Francois Fillon, Italy's Silvio Berlusconi, Ukraine's Julia Tymoshenko and Sweden's Fredrik Reinfeldt are among those expected to attend ceremonies later in the day.
It is the first time a senior Russian official has attended the war memorial ceremonies in Poland.
The ceremony comes as Sergei Lavrov, Russia's foreign minister, expressed fury over moves by former Communist states to equate the dictatorship of Joseph Stalin, the WWII leader of the Soviet Union, with that of the Nazi government.
|World War II began when Nazi Germany invaded Poland in 1939 [AFP]
"Even during the Cold War no-one ever tried to put the Nazi regime on one level with the Stalin dictatorship," Lavrov said in an article published in Rossiiskaya Gazeta on Tuesday.
"It never occurred to anyone to equate the Nazi threat, which meant the annihilation of whole peoples, and the politics of the Soviet Union, which was the only force able to resist the war machine of Hitler's Germany."
His comments were a reaction to a resolution by the Organisation for Security and Co-operation in Europe (OSCE) parliamentary assembly in July, proposed by Lithuania and Slovenia, that likened Nazism and Stalinism.
The OSCE resolution also proposed making August 23, the date of a pact signed between Nazi Germany and the Soviet Union on the eve of the German invasion of Poland, as a memorial day for victims of Nazism and Stalinism.
But Poland and other Baltic states say Stalin bears direct responsibility for the outbreak of war for agreeing to a pact with Germany on the eve of the war, that carved up the country.
'Question the truths'
The Nazi-Soviet Non-Aggression Treaty, known as the Molotov-Ribbentrop pact, led to the partition of Poland between Germany and the Soviet Union.
Lech Kaczynski, Poland's president, wrote in an article published on Tuesday: "[We need] to oppose attempts to write history anew, to question the truths of World War Two, the scale of the casualties of Nazism and also of total communism."
However Putin, speaking before Tuesday's ceremonies, offered a more conciliatory note, saying that Russian ties with Poland had improved since Tusk came to power.
The Russian prime minister said the Kremlin had found in the Polish government "colleagues and partners that we can work with".
Nazi Germany sparked World War II when it invaded Poland on September 1, 1939.
Around six million Poles were killed in the War, half of them Jews, while its main cities were left in ruins.
In proportion to its size, Poland suffered more damage and casualties than any other country involved.