WARSAW — The mayor of Gdansk, Poland, a leading liberal critic of the populist, right-wing national government, died on Monday after being stabbed at a public charity concert Sunday night, the minister of health told reporters.
Mayor Pawel Adamowicz, 53, the mayor of the northern port city since 1998, was known as a supporter of gay rights, and he had campaigned for the rights of immigrants in a country whose governing party has leaned heavily on anti-immigrant rhetoric.
“It was impossible to win against everything that had happened to him,” Lukasz Szumowski, the health minister, said of the stabbing. “God rest his soul.”
The attack stunned a nation that is increasingly divided politically, and rallies scheduled for Monday night to condemn violence and hate speech were expected to draw large crowds.
Police officials said the assailant was arrested at the scene, and described him as a 27-year-old, mentally disturbed man with a history of violence and no clear political motive.
The attack took place at the nation’s largest charity event, held every year to raise money for medical equipment. It was just before 8 p.m. Sunday, and tens of thousands of people had gathered for a concert to promote the charity.
A countdown had begun to signal the start of an extravagant laser light show called “Light to the Sky.” One second before fireworks were set to explode, a young man burst onto the stage and stabbed Mr. Adamowicz several times, including in the heart.
The assailant then circled the podium waving a black knife and screamed that he had been thrown in jail under Civic Platform, the political party to which the liberal mayor once belonged.
“That’s why I killed Adamowicz,” the man shouted.
Mr. Adamowicz’s injuries included “a deep wound to the heart, a wound to diaphragm and other injuries of internal organs,” doctors at Medical University of Gdansk said. Despite their efforts to save him, he died Monday afternoon.
As the city went into mourning, Poland grappled with the question of whether the toxic and aggressive tone of the country’s political debate could have instigated the attack.
The conservative governing party was quick to condemn the assault.
“We usually disagree with Mr. Mayor Pawel Adamowicz when it comes to political views on how to lead Poland, but today we are with him and his loved ones unconditionally, as are — I hope — all of our compatriots,” President Andrzej Duda wrote on Twitter after the attack.
The government, of which Mr. Adamowicz was a fierce critic, dispatched a plane to London to fly the mayor’s wife back to Gdansk.
Aleksandra Skorupka-Kaczmarek, a deputy mayor of Gdansk, said Monday morning that Mr. Adamowicz’s friends and family were struggling to understand what had happened.
“We’re all asking the question of how one can attack an innocent man,” she said. “Let’s eliminate the aggression from our public life, political life. Let’s not escalate this violence. Please, don’t use this tragedy for political and ideological ends.”
After the attack, the police arrested a man who had threatened on Twitter the mayors of two other Polish cities, Wroclaw and Poznan. Recent elections have shown a deep and widening political divide between voters in rural areas and small towns, who largely support the governing Law and Justice party, and those in larger cities, who mostly oppose the government.
Ahead of local elections last October, All-Polish Youth, a far-right organization, issued fake death certificates for 11 liberal politicians, mostly associated with Civic Platform, including Mr. Adamowicz. The prosecutor’s office refused to investigate the initiative, calling it “an expression of opinion,” not “incitement of hatred.”
Mr. Adamowicz opposed that decision, and said just last week: “This wasn’t a regular expression of opinion; they truly crossed the line. I’m not going to leave it like this.”
Residents of Gdansk and Warsaw planned to hold rallies Monday afternoon against violence and the use of aggressive language by Polish politicians.
Mr. Duda called for a demonstration on Tuesday and invited leaders of all major political parties to join him.
The charity event on Sunday is Poland’s biggest annual fund-raiser, organized since 1993 by the Great Orchestra of Christmas Charity. Over the years, the organization has raised about $266 million to provide medical equipment to Poland’s underfunded hospitals.
The event this year was set to break last year’s record for money raised.
“This is a wonderful time of spreading good,” Mr. Adamowicz said moments before he was attacked. “You are all wonderful. Gdansk is the most amazing city.”
After the stabbing and claims that the event had insufficient security, the president of the foundation, Jerzy Owsiak, resigned.
“This hatred, which is deep-seated in people, exploded in an extreme way,” Mr. Owsiak, a fierce critic of the governing party and a frequent object of attacks from right-wing politicians, told reporters. “I’ve been fighting those who threaten me for 25 years. Poland’s justice system and police are completely helpless.”
Police officials said that the suspect had recently been released from prison, after serving more than five years for several bank robberies. They said the man, identified only as Slawomir. W., had gained access to the stage with a media badge, but it was not clear where he got it.
Mr. Adamowicz, who is survived by his wife, Magdalena, and two daughters, preached the value of tolerance and held up his city as a progressive model of integrating immigrants. He participated in Gay Pride events in Gdansk and expressed solidarity with the city’s Jewish community after stone-throwing vandals broke windows in a local synagogue.
He studied law at the University of Gdansk. As a student, he participated in the pro-democracy resistance against the Communist regime in his city.