Germany's 1939 invasion of Poland opened the ‘most tragic chapter’ in European history, German Chancellor Angela Merkel has said at a ceremony marking the 70th anniversary of the start of World War II.

‘The war unleashed by Germany resulted in immeasurable suffering to many peoples - years of deprivation of rights, of humiliation and destruction,’ she said in the Polish city of Gdansk.

European leaders remembered the victims of World War II at ceremonies today.

Veterans of the war joined Poland's leaders for the ceremony in Gdansk, the site of the first battle on 1 September 1939, when a German ship opened fire on a Polish base.

Russian Prime Minister Vladimir Putin was among the leaders who attended today's ceremony.

At the ceremony, Mr Putin and Polish Prime Minister Donald Tusk agreed that their countries' historians should work more closely to uncover darker parts of their shared past, which still cloud relations between the two countries.

Russia and former satellites such as Poland are at loggerheads over the role of Soviet dictator Josef Stalin in 1939, when he clinched a non-aggression pact with Nazi Germany that opened the way for the invasion of Poland and world war.

'If we are going to speak objectively about history we must understand it does not have just one colour. It was diverse and a huge number of mistakes were made by all sides,' Mr Putin told a news conference after talks with Mr Tusk.

'And all these actions created the conditions for the large scale aggression by Nazi Germany.'

Russians are deeply proud of their country's victory over Hitler in 1945, but Poles, Balts and others say Stalin also bears direct responsibility for the outbreak of war, for carving up Poland with Hitler and also annexing the Baltic states.

However Mr Putin cited efforts by Britain and France to appease Hitler in 1938, resulting in their acceptance of the destruction of Czechoslovakia, as well as Poland's own seizure of a strip of Czech territory shortly before it too faced German invasion.

Mr Putin and Mr Tusk have now agreed to set up joint teams of historians to study the murder of Polish officers in a forest at Katyn in the western Soviet Union in 1940 - an event which for Poles symbolises what they see as Stalin's treachery

For decades, Moscow blamed the deaths on the Nazis, but after the fall of the Soviet Union it acknowledged they had been shot on Stalin's orders.

Despite tensions, a ceremony at dawn this morning saw Polish President Lech Kaczynski and Mr Tusk pay tribute to the victims of the conflict, which claimed at least 50 million lives.

Ms Merkel, the prime ministers of Italy and France, and Britain's foreign secretary are also among the guests attending the commemorative events.

Poland lost about a fifth of its population, including the vast majority of its three million Jewish citizens during World War II.

After the war, it remained under Soviet domination until 1989.

Some 27m Soviet citizens perished in the war after Hitler reneged on his pact with Stalin and invaded the Soviet Union in 1941.