The flag was raised on the clock tower in a ceremony attended by President Andrzej Duda and First Lady Agata Kornhauser-Duda.
After the ceremony, the presidential couple handed out red-and-white flags to Warsaw residents, public broadcaster Polish Radio’s IAR news agency reported.
The Polish president has said he is happy to see that Poles treat their national colours “with respect and pride.”
The Polish Prime Minister's Office said in a tweet on Tuesday that "the flag of Poland consists of two horizontal stripes: white and red."
It added that "white and red are Polish national colours and a symbol of patriotic values, which unite all Poles."
Polish Foreign Minister Zbigniew Rau said in a Twitter post that "the white-and-red flag is a symbol of our identity and unity."
The Polish defence ministry said in a message that the Flag Day celebration aimed to promote respect for the national colours and symbols as well as "a sense of community and national pride."
Polish Deputy Prime Minister and Culture Minister Piotr Gliński has said that the national flag is a powerful unifying symbol and encouraged people to focus on "things that unite us rather than those that divide us.”
Poland's Flag Day is a relatively new tradition that was established in 2004.
People across the country were being encouraged to display national flags in front of their homes.
Day of Poles Abroad
As well as Flag Day, May 2 is the Day of Poles Abroad, and officials have sent greetings to compatriots living around the world.
May 2 marks Flag Day in Poland, when many people display the national colours and are encouraged to make patriotic statements.
Meanwhile, May 3 is Constitution Day, celebrating a historic constitution the country adopted on May 3, 1791.
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Source: IAR, PAP, TVP Info