The 30-foot (nine-meter) tall white granite cross stands on the site where John Paul II delivered a Mass in 1979 in then-communist Poland.
The sermon is credited by many with inspiring the country's nascent pro-democracy groups and giving rise to the Solidarity freedom movement that helped topple communist rule in 1989.
"We Poles know that the overthrow of communism did not start in 1989 but 10 years earlier, here on this square with the words of John Paul II," Warsaw mayor Hanna Gronkiewicz-Waltz told the thousands of faithful who attended the ceremony and later Mass.
"From today forward, in the heart of Poland and Warsaw ... will stand a cross that is a symbol of faith, perseverance and hope."
Warsaw Archbishop Kazimierz Nycz also recited a short prayer and blessed the monument with holy water.
John Paul II, who died in 2005, is still a highly revered figure in his homeland.
The pontiff's encouragement to his countrymen of peaceful challenge to the communist regime is credited by many with hastening its demise.
This year's anniversary of his first pilgrimage comes as Poland also celebrates the 20th anniversary of the country's first partly free elections that marked the end of communist rule.
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