Pope Benedict XVI has celebrated open Mass in the centre of Warsaw, with more than 200,000 people in attendance.
The Mass had echoes of John Paul's emotional address in 1979
The pontiff arrived in Pilsudski Square in driving rain, to the cheers of sodden but resolute crowds waving the flags of Poland and the Vatican.
In his sermon the German-born Pope paid tribute to his Polish mentor and predecessor, John Paul II.
He later visited the town of Krakow, where John Paul was archbishop, and Poland's holiest shrine at Czestochowa.
Speaking from a window at the Krakow's archbishop's palace, where John Paul II had lived until 1978, Pope Benedict XVI urged a crowd of pilgrims to pray for the beatification of his "beloved predecessor".
"This prayer supports those working on his cause," he said, drawing applause from the crowd.
He later prayed before a gathering of more than a quarter of million people at a shrine in Czechostowa in southern Poland.
The Jasna Gora shrine houses an ancient portrait of the Virgin Mary, known as the Black Madonna.
On Sunday, the Pope will visit the Auschwitz death camp to pray for reconciliation between nations and faiths.
Some 220,000 people are estimated to have crammed into Pilsudski Square to watch the Pope perform Mass on Friday.
Church bells pealed as the Pope, accompanied by 120 priests and bishops who celebrated the mass with him, ascended a metal, three-tiered platform for the ceremony.
In his sermon, the Pope defended the Church's traditional values, warning against those "seeking to falsify the Word of Christ and to remove from the Gospel those truths which in their view are too uncomfortable for modern man."
German-Polish reconciliation is a theme of the Pope's visit
"They try to give the impression that everything is relative: even the truths of faith would depend on the historical situation and human evaluation," he said. "Yet the church cannot silence the spirit of truth."
He also paid tribute to his predecessor's words on the same spot during a visit in 1979, which challenged the communist authorities and was seen by many as a rallying call to his countrymen to stand up to communism - which was eventually toppled a decade later.
"Before our eyes, changes occurred in entire political, economic and social systems," he said. "People in various countries regained their freedom and their sense of dignity."
Pope Benedict has said he has "come to follow in the footsteps" of John Paul II.
Poles are unlikely to have the same emotional connection with the new Pope - but as a trusted aide and close friend of John Paul he is the next best thing, the BBC's Adam Easton reports from Warsaw.
Since his election a year ago, the Pope has been to southern Italy and to his native Germany - but the Polish visit was the first organised on his own initiative.
"This is not just a sentimental journey, but a journey of faith," Pope Benedict said on his arrival in Poland on Thursday.
"I so wanted to visit the country of my beloved predecessor."
On the way from the airport to St John's Cathedral in Warsaw he made a detour in his Popemobile to pass by the site of the 1943 Warsaw ghetto uprising, honouring the Jews who resisted the Nazi occupation.
At the monument he briefly made a sign of blessing to more than 40 elderly Poles who risked their lives helping Jews during the war.
On visiting Auschwitz, Pope Benedict, who was once a reluctant member of the Hitler Youth, will walk, not drive, through the notorious death camp gates, and will refrain from speaking German during the visit.