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WARSAW, Poland — The mayor of a city in Poland died after he was stabbed in the heart on stage Sunday during the finale of a large charity event.

Doctors operated for five hours on Gdansk Mayor Pawel Adamowicz, who was attacked Sunday by an ex-convict who rushed onto the stage with a knife, shouting it was revenge against a political party Adamowicz had previously belonged to.

Adamowicz — who has been the city's mayor for more than 20 years — grabbed his belly and collapsed in front of the audience at the highly popular annual fundraiser organized by the Great Orchestra of Christmas Charity.

Polish blood donors came out in droves in a bid to save his life, but Poland's health minister said Adamowicz, 53, died of his wounds on Monday.

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The assailant shouted from the stage that he had been wrongly imprisoned under a previous government led by Civic Platform, a party to which the mayor formerly belonged. He said that his name was Stefan and that "I was jailed but innocent. ... Civic Platform tortured me. That's why Adamowicz just died."

Police said the suspect, a 27-year-old Gdansk resident who was recently released from prison where he had served a term for bank robberies, has been detained and is awaiting questioning by prosecutors.

Spokeswoman for the ruling Law and Justice party Beata Mazurek said the attack should be "absolutely condemned by all, regardless of what side of the political stage they are on."

She insisted politicians in Poland need "greater responsibility for words, for deeds" because "there is no shortage of madmen on both sides" of the political spectrum.

TV footage showed Adamowicz on stage Sunday with a sparkler in hand telling the audience that it had been a "wonderful day" of the charity collecting money across Poland and abroad for cash-strapped hospitals. Then the attacker came toward him.

Gdansk's Mayor Pawel Adamowicz, second right, speaks during the 27th Grand Finale of the Great Orchestra of Christmas Charity in Gdansk, Poland, on Jan. 13, 2019. Bartosz Banka / Agencja Gazeta via Reuters

European Council President Donald Tusk, a former Polish prime minister who co-founded Civic Platform and is from Gdansk, wrote on Twitter: "Let's all pray for Mayor Adamowicz. Pawel, we are with you."

Adamowicz, had been mayor of Gdansk, a Baltic port city, since 1998. He was part of the democratic opposition born in that city under the leadership of Lech Walesa during the 1980s. After leaving Civic Platform, he was re-elected to a sixth term as an independent candidate in the fall.

A man is held on the ground by security personnel after he attacked the mayor of Gdansk, Poland, on Sunday during a charity event. Piotr Hukalo / AFP - Getty Images

As mayor, he has been a progressive voice, supporting LGBT rights and tolerance for minorities. He marched in last year's gay pride parade, a rare action for a mayor in Poland.

He also showed solidarity with the Jewish community when the city's synagogue had its windows broken last year, strongly denouncing the vandalism.

The Great Orchestra of Christmas Charity raises money for state-of-the-art medical equipment for Poland's cash-strapped health care system, mostly for children but also for the elderly.