Gdansk mayor Pawel Adamowicz has died from stab wounds a day after being attacked on stage at a charity event, Poland's health minister said.
Lukasz Szumowski said the doctors who were fighting to save Mr Adamowicz's life informed him the mayor had died.
Mr Adamowicz was hospitalised for serious abdominal injuries and had five hours of surgery.
Prosecutors said the man accused of stabbing the mayor is being investigated on suspicion of attempted murder and will also be subjected to a psychiatric examination.
They are also looking into the level of security at Sunday's public event, which was the 27th annual festive collection for a popular charity.
Interior minister Joachim Brudzinski described the incident as "an act of inexplicable barbarity" on Sunday.
Mayor of London Sadiq Khan described Mr Adamowicz's death as a loss "for all of us who value public service".
"Devastating loss for the people of Gdansk, for Mayor Adamowicz’s friends and family and for all of us who value public service, progressive politics and open, accessible democracy.
"London’s thoughts, prayers and solidarity are with Gdansk on this sad day."