Private broadcaster TVN24 cited Interior Minister Joachim Brudziński as saying that mayor Paweł Adamowicz, a well-known local politician, had to be resuscitated and was in a “serious” condition.
The attack, which took place during a high-profile, long-running fundraiser called the Great Orchestra of Christmas Charity, caused shock and disbelief in Poland.
TVN24 reported that Adamowicz was wounded by a knifeman who ran shouting onto a stage. It added that the mayor was taken to a local hospital, where he was undergoing an operation.
Brudziński said: “I can’t find the words, an explanation, for this kind of act. Especially since the finale of the Great Orchestra of Christmas Charity has for the last 27 years always been associated with good, with something joyful, positive.”
PM condemns attack
Prime Minister Mateusz Morawiecki condemned the attack. He wrote on Twitter: “We are with Mayor Adamowicz in our thoughts and prayers.”
The attacker reportedly shouted that he had unfairly spent time in prison when the previous Civic Platform-led government was in power, according to TVN24. He claimed he had been "tortured" by the Civic Platform.
After the stabbing, the man made an apparent gesture of triumph and strutted around the stage, media reported.
The knifeman was disarmed at the scene, TVN24 said.
Public broadcaster Polish Radio reported that investigators were collecting evidence at the site.
Police said they had detained a 27-year-old local resident.
Messages of support
In an unusual display of unity in Poland, messages of support for Adamowicz flowed in from politicians from different ends of the political spectrum.
Earlier on Sunday, some 120,000 volunteers took to the streets across the country and in many locations abroad to raise funds during the Great Orchestra of Christmas Charity drive's finale, an event that draws huge support from the public and celebrities every year.
This year’s proceeds were to go towards medical equipment at children's hospitals in Poland.