President Andrzej Duda said in a video message that Poland was the first country to recognize Ukrainian independence in 1991.
He added that Poland "today stands in solidarity" with its eastern neighbour as Ukraine "fights to defend its freedom" from Russia.
Duda said: "I know that you will win this war. We, the people of Poland, believe that a free Ukraine, rebuilt from wartime destruction, will enjoy prosperity in the European community."
He added: "Long live Ukraine, long live Poland."
'We are proud to call ourselves your allies': Polish PM
Poland's Prime Minister Mateusz Morawiecki said in a Twitter post: "Today is the 31st anniversary of the declaration of Ukrainian independence. Poland was the first country to recognize the independence of Ukraine."
He added: "On this day, I have one wish for Ukraine that Ukrainian soldiers will celebrate independence at home with their families next year."
"Poland looks at your struggle with admiration," Morawiecki also said in his message, as quoted in English by his office. "We are proud to call ourselves your allies, your friends. Your protect yourself and your families, but also Poland and Europe."
"We are with Ukraine for better and for worse," Morawiecki declared. "We support you through deliveries, work in international forums and, above all, by sheltering Ukrainian women and children who escaped from the war."
'No empire and no authority can destroy a free people'
"Our nations have come a long way and are on the same side today: the side of freedom," he also said. "Together we guard the heritage of European civilization. We believe that no empire and no authority can destroy free people."
'I wish all Ukrainians strength, peace and perseverance': Polish FM
Polish Foreign Minister Zbigniew Rau wished Ukrainians "strength, peace and perseverance in their fight for freedom and independence."
He tweeted: "For 31 years, Ukraine has proudly celebrated Independence Day.
Although this year's celebration, six months since the beginning of Russia's invasion, is different, you are not alone."
'Six months of Russian shame, Ukrainian resistance': Polish foreign ministry
The Polish foreign ministry in a tweet voiced "solidarity with the Ukrainian nation in its fight with the aggressor."
It said: "In our own homes and all over the world, Poland and the Polish people support and will continue to support their friends who are bravely fighting for the freedom of us all."
In a separate tweet, the Polish foreign ministry said that "six months ago, the eyes of the world turned to Ukraine and its heroism. From that moment on, the world we knew has changed."
The Polish foreign ministry added: "Six months of Russian shame. Six months of Ukrainian resistance, unity, bravery, brotherhood and hope for a better, peaceful future."
Ukrainian President Volodymyr Zelensky said on Wednesday that his country was "reborn" when Russia invaded it six months ago and vowed that Kyiv would never give up its fight for freedom.
Wednesday marks 31 years since Ukraine regained its independence from the Soviet Union in 1991 and also exactly six months since Russia invaded Ukraine on February 24.
Polish President Andrzej Duda on Tuesday visited Kyiv to mark the anniversary of Ukrainian independence and pledge more aid to the war-torn country.
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Source: IAR, PAP, ukrinform.net, Reuters