Tens of thousands accompanied the coffin of the late Gdansk Mayor Pawel Adamowicz to the church for his funeral on Saturday.
The march escorted the black hearse past his old schools and his childhood home.
Thousands of people had already paid their last respects at the European Solidarity Centre where the coffin lay for 24 hours.
Top Polish politicians and invited guests from abroad will attend the mass at St Mary Basilica (noon), including the president of the European Council Donald Tusk, who was a personal friend.
The 53-year-old was stabbed on Sunday during a charity concert and died on Monday in hospital.
Flags have been flying at half-mast and Saturday is a national day of mourning.