The mayor of the Polish city of Gdańsk has been seriously hurt after being stabbed on stage at the country's biggest charity event, officials say.
Pawel Adamowicz, 53, had to be resuscitated after the attack and is in a critical condition in hospital.
The 27-year-old suspect, who has a criminal record, has been detained.
He reportedly shouted that he had been wrongfully imprisoned under the previous government, led by the mayor's former party.
The incident took place at the Great Orchestra of Christmas Charity event, which was attended by hundreds of people.
The suspect, who was released from prison recently, had access to the scene using a media badge, Polish media say.
The stabbing, during an event that raises money for children's hospitals, has shocked Poland, the BBC's correspondent in Warsaw said.
Mr Adamowicz was a well-known political figure who has been mayor of Gdansk, a Baltic port city, for 20 years, they said.
Interior Minister Joachim Brudzinski said the attack was "an act of inexplicable barbarism".
Prime Minister Mateusz Morawiecki said on Twitter: "The attack on life and health of Paul Adamowicz is worthy of the highest condemnation."
Also on Twitter, President Andrzej Duda said: "I pray for his return to health and full strength."