A day of commemorations is taking place in Poland to mark the 70th anniversary of the outbreak of World War II.
Leaders from 20 countries took part in a wreath-laying ceremony near Gdansk, on Westerplatte peninsula.
A dawn ceremony there had marked the time when a German battleship fired the first shots on a Polish fort in 1939.
President Lech Kaczynski added to a row with Russia over responsibility for the war, saying his country had received a “stab in the back”.
In a news conference with his Polish counterpart, Russian Prime Minister Vladimir Putin acknowledged there were “problems” in the two countries’ history which needed to be analysed.
Relations between Poland and Russia are currently thorny, partly because of differing historical interpretations of events at the start of the war.
Mr Putin added that the pair should “rise above the problems of the past… and solve the problems of the future”.
He went on to talk about trade and energy co-operation between the two.
Earlier, Mr Kaczynski and his prime minister Donald Tusk joined war veterans beside a monument to the heroes of Westerplatte at 0445 (0245 GMT).
The ceremony marked the exact time on 1 September 1939 when the German battleship Schleswig-Holstein opened fire at point-blank range on the fort.
At the same time, the German Wehrmacht invaded Poland from east, west and south. The attacks triggered Britain and France’s declaration of war against Germany two days later.
Poles, though, have long seen the Nazi-Soviet Non-Aggression Treaty, signed a week before war started, as the starting gun for the German invasion, says the BBC’s Jonny Dymond in Gdansk.
Just two weeks later, in mid-September 1939, the Soviet armies occupied eastern Poland.
Speaking at the ceremony, Mr Kaczynski said: “On 17 September… Poland received a stab in the back… This blow came from Bolshevik Russia.”
He added: “Glory to the heroes of Westerplatte, glory to all of the soldiers who fought in World War II against German Nazism, and against Bolshevik totalitarianism.”
In his address, Mr Tusk said the lessons of history should not be forgotten.
“We remember because we know well that he who forgets, or he who falsifies history, and has power or will assume power will bring unhappiness again like 70 years ago,” he said.
At the time of the attack by the Schleswig-Holstein – which was moored in the Polish harbour on a friendship visit – Gdansk was known as the free city of Danzig.
The 182 Polish troops defending the Polish fort were expected to resist for about 12 hours. Despite coming under fire from the air, sea and land, they held out against a force of more than 3,000 Germans for seven days.
According to a survey published on Monday, Westerplatte is the most important symbol of Polish resistance in the whole of the war.
A wreath-laying ceremony will take place later in the day and, of the speeches expected throughout the ceremonies, it is Mr Putin’s which is the most keenly anticipated in Poland.
According to the historian Professor Pawel Machcewicz, the Poles are expecting some sort of gesture from Mr Putin.
Two weeks after the German invasion, the Red Army invaded and annexed eastern Poland under terms agreed in the secret protocol of a Nazi-Soviet pact.
In early 1940, the Soviet secret services murdered more than 20,000 Polish officers in the forests around Katyn. For 50 years Moscow blamed the Nazis and only admitted responsibility for the crime in 1990.
Russian courts have ruled that Katyn cannot be considered a war crime and Moscow is still refusing to declassify documents about the massacre.
The temperature was raised further this week with accusations broadcast on Russian state TV which implied the USSR was justified in its invasion of Poland because Warsaw had been conspiring with Hitler against Moscow.