Polish leaders, diplomats and veterans gathered early on Tuesday in Westerplatte near Gdansk, in northern Poland, to mark the 70th anniversary of the outbreak of World War II.

Foreign leaders from 20 countries, including former belligerents Germany and Russia, were to meet later in the day in Gdansk for the main ceremony.

Exactly 70 years ago, a Nazi German battleship opened fire on a Polish fort in Westerplatte on the Baltic Sea, triggering the start of the world’s most brutal and costly conflict. The fort's 180 defenders put up a heroic resistance for a week against 3,500 German soldiers.

The ceremonies began against the backdrop of a bitter dispute between Moscow and Warsaw over who should bear responsibility for the start of World War Two.

After a meeting with his Polish counterpart Donald Tusk, Russian Prime Minister Vladimir Putin struck a conciliatory tone as he recalled that “Russians and Poles had fought side by side against a common enemy” during a conflict in which Russians “had looked upon Poles as their brothers in arms”.