In the letter, statesman Józef Piłsudski – who is considered the father of Poland's independence – said he was the chief commander of Poland's armed forces and that he wanted "to notify governments and countries about the existence of an independent Polish state".
The letter was sent on November 16, 1918, to the president of the United States and to the governments of Britain, France, Italy, Japan, Germany.
People from those same countries are rereading the letter on November 11, 2018, as Poland marks exactly 100 years since Piłsudski arrived in Warsaw after being held prisoner in Germany announcing Poland's independence after more than 120 years of partitions and foreign rule.
Meanwhile, in Poland, 20,000 people donned red and white jerseys to form a Polish flag before starting a 10-kilometre Independence Run.
At exactly 11:11 am, the time reflecting the date Poland celebrates Independence Day, the runners started their race which they said was their way of paying tribute to the Poles before them who fought for Poland's independence.
Others rode on a "freedom train" made up of a steam engine and wagons dating back to the first half of the 20th century.
The train featured an exhibition of militaria and historical reactors in military uniforms dating back to WWI, with one actor portraying marshal Piłsudski.
Official events included a Mass in Warsaw, dedicated to Poland's freedom and independence, which was attended by the country's prime minister and president as well as other top politicians and a few hundred Varsovians.
A letter from Pope Francis, the head of the Catholic Church, was read during the Mass.
Official events continued with a ceremonial changing of the guard at Warsaw's Tomb of the Unknown Soldier.
At the Tomb of the Unknown Soldier, Polish President Andrzej Duda is to give a speech before the National Anthem is sung.
Later, the president is to take part in the inauguration of an Independence Festival, which will feature traditional song and dance, before posthumously awarding 25 famous Poles the Order of the White Eagle, Poland's top civilian state distinction.
On Sunday afternoon, a White-and-Red March is to take to the streets of Warsaw.
To mark the 100th anniversary of Poland's independence, public broadcaster TVP said it would show the pilot episode of a new television series about a young Piłsudski. (vb/di)