The flag was raised on the clock tower in a pared-back ceremony amid the coronavirus pandemic.
Earlier in the day, Defence Minister Mariusz Błaszczak led a military event at Warsaw's central Piłsudski Square.
Defence Minister Mariusz Błaszczak during ceremonies at Warsaw's Piłsudski Square. Photo: PAP/Piotr Nowak
Poland's Flag Day is a relatively new tradition - it was established in 2004.
People across the country were being encouraged to display national flags in front of their homes.
Polish Diaspora Day
As well as Flag Day, May 2 is the Day of Poles Abroad, and Polish Foreign Minister Zbigniew Rau has sent his best wishes to his compatriots living in other countries.
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Source: PAP