Polish mayor Pawel Adamowicz died at the hospital after being stabbed on stage at a charity event.
Adamowicz, mayor of Gdansk, was assaulted in front of hundreds of people in town on sunday at an event for the benefit of the Grand Orchestra of Christmas charity, which raised funds for the purchase of hospital equipment.
He underwent five hours of operation for heart and abdominal wounds, while residents lined up to donate blood, but Health Minister Lukasz Szumowski told reporters on Monday : "We could not win.
"It was not possible to overcome everything that had happened to him.He's resting in peace."
Mr. Adamowicz was attacked in front of hundreds of spectators
Police arrested a 27-year-old man with an alleged criminal record of attempted murder and is still in police custody.
Investigators said he appeared to have mental health issues and that he had access to the scene with a badge.
According to the Polish television channel TVN, he shouted from the theater claiming that he had been improperly imprisoned by the previous government before attacking his victim with a 14.5 cm knife.
The suspect – who, according to the authorities, has already been convicted of involvement in bank robberies and served a five-and-a-half-year sentence – will be subjected to a psychiatric examination.
Polish Prime Minister said that the attack was "worthy of the highest sentence"
After the announcement of his death, the President of the EU Council, Donald Tusk, tweeted: "Pawel Adamowicz, Mayor of Gdansk, a man of solidarity and freedom, a European, my good friend, has been murdered, that he rests in peace. "
Polish Interior Minister Joachim Brudzinski on Sunday condemned the attack on the 53-year-old mayor as an act of "inexcusable barbarity".
Moments before the incident, Mr. Adamowicz – who was in his sixth term as mayor – posted a picture of the scene point of view on his Instagram account.
The photo showed members of the public wielding bright white lights during the Lights To Heaven event, which had raised funds for the purchase of medical equipment for the Polish health system.
The politician was mayor of Gdansk for more than 20 years and was part of the democratic opposition born in the city during the reign of Lech Walesa in the 1980s.
He was perceived as a progressive voice in the country, defending LGBT rights and tolerance towards minority groups, and also advocating for Gdansk care for Syrian children wounded for medical treatment
Anti-violence rallies are planned throughout the country in response to the attack.