Warsaw - Poles marked the 69th anniversary of the outbreak of World War II on Monday with a ceremony at Westerplatte, the site of the war's first battle after Germany invaded Poland in 1939.

Sirens were sounded at 4:45 am near the Baltic coastal city of Gdansk, marking the exact time a German battleship first fired at the Polish garrison.

Prime Minister Donald Tusk took part in the ceremony, speaking on the importance of remembrance.

"Why do we meet with such determination, why do we commemorate such dates," Tusk said. "We've built, and we will build, our national identity on the remembrance of our heroes and such events."

The nearby city of Gdansk is also the site of the shipyards where labor union Solidarity, lead by Lech Walesa, challenged the country's communist regime in the 1980s.

Tusk said the events will be remembered with a Westerplatte museum, a World War II museum and a European Solidarity center, which are slated to be built as a complex.

Revitalizing Westerplatte is especially important with the upcoming 70th anniversary of the war's outbreak, said Slawomir Nowak, a top aide to the Prime Minister.

"On September 1, 2009, all of Europe must remember," Nowak said, "where the nightmare of World War II began."

The Battle of Westerplatte marked the beginning of World War II in Europe.

Polish soldiers repelled for seven days repeated attacks on the garrison from German forces. (dpa)