Pawel Adamowicz, the mayor of Gdansk, had been on stage at the Light to Heaven charity event when he was attacked.
Pawel Adamowicz was attacked with a sharp tool during the fundraising finale organized by the Great Orchestra of Christmas Charity.
Interior Minister Joachim Brudzinski said Adamowicz was hospitalized in very serious condition.
Mr Adamowicz later underwent surgery, deputy mayor Piotry Grzelak, was quoted as saying by the PAP news agency.
The broadcaster reported that the attacker was detained.
A man is held on the ground by security personnel after he attacked the mayor of Gdansk during a charity event. A police spokesman, Mariusz Ciarka, said the attacker appeared to have mental problems and gained access to the area with a media badge.
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The incident took place at the Great Orchestra of Christmas Charity event, which was attended by hundreds of people.
Footage of a man on stage has emerged online, claiming to show him brandishing a knife.
Overseas officials including EU President Donald Tusk, European Commission Vice President Frans Timmermans and mayor of London Sadiq Khan also expressed their support for Adamowicz. Owsiak referred to being personally depicted in a defamatory manner in an animation that ran on state TV last week and also had anti-Semitic overtones. A Star of David was on one of the banknotes. He was part of the democratic opposition born in that city under the leadership of Lech Walesa during the 1980s.
He is seen as a progressive voice and has supported LGBT rights and tolerance for minorities. He showed solidarity with the Jewish community when the city's synagogue had its windows broken a year ago, strongly denouncing the vandalism.
The stabbing, during an event that raises money for children's hospitals, has shocked Poland, the BBC's correspondent in Warsaw said.
The shocking attack, previously unheard-of in the European city, has drawn immediate condemnation, even from the mayor's biggest political rivals.
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