The popular liberal mayor of the Polish port city of Gdansk died on Monday after he was stabbed during a charity event the evening before by an ex-convict who stormed onstage and said it was revenge against a political party the politician once belonged to.

Pawel Adamowicz, 53, died as a result of wounds to the heart and abdomen in spite of efforts to save him that involved a five-hour operation and blood transfusions, Health Minister Lukasz Szumowski said.

“The fight for his life has been lost,” Szumowski said.

The assassination of Adamowicz, a six-term mayor who often mingled freely with citizens of his city, sent Poland into shock.

Even before his death was announced, rallies against violence were being planned to take place across Poland in the evening. In Gdansk, the city flag was lowered to half-staff and a Mass was planned for later in the day.

The right-wing ruling Law and Justice party faced accusations from its critics that an atmosphere of hatred against Adamowicz and others liberal political opponents helped instigate the attack.

Government officials appeared to be pushing back against that accusation, strongly denouncing the attack and stressing that the 27-year-old perpetrator had a history of violent bank robberies and possible mental illness.

The ex-convict who rushed onto the stage with a knife Sunday and stabbed Adamowicz shouted that it was revenge against Civic Platform, which Adamowicz belonged to for many years.

The assailant shouted from the stage that he had been wrongly imprisoned under a previous government led by Civic Platform. He said his name was Stefan and that “I was jailed but innocent. … Civic Platform tortured me. That’s why Adamowicz just died.”

Deputy Chief Prosecutor Krzysztof Sierak said there are “doubts” as to the mental state of the attacker, who used a 14.5-centimetre (5.5-inch) knife on Adamowicz, and that two psychiatrists will examine him. He had served 5 1/2 years in prison and was released toward the end of last year.

Adamowicz, who has been the city’s mayor for more than 20 years, grabbed his belly and collapsed in front of the audience during the “Lights to Heaven” fundraiser organized by the Great Orchestra of Christmas Charity.

The attack triggered an outpouring of solidarity, with many people donating blood in Gdansk on Monday. Some said they were given time off work to help save Adamowicz.

The 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 spectrum 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 scale.

Ruling authorities also sent a government plane to transport the mayor’s wife, who had been travelling, from London back to Gdansk.

The government’s critics, however, said that they believed that animosity voiced against Adamowicz by ruling party officials, sometimes carried on state television, as well as by extremists, played a role.

Adamowicz was part of the democratic opposition formed in Gdansk 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.

As mayor, he was a progressive voice, supporting sex education in schools, LGBT rights and tolerance for minorities. He showed solidarity with the Jewish community when Gdansk synagogue had its windows broken last year, strongly denouncing the vandalism.

Adamowicz also advocated bringing wounded Syrian children to Gdansk for medical treatment, a plan, however, blocked by the Law and Justice government. After he took that stand, a far-right group, the All-Polish Youth, issued what they called a “political death notice” for Adamowicz.

The last politically motivated attack in Poland was in 2010 in Lodz when a man shouting that he wanted to kill Law and Justice party leader Jaroslaw Kaczynski fatally shot an aide to one of the party’s European Parliament lawmakers.

Kaczynski, at the time an opposition leader, blamed the attack on an “atmosphere of hate” under Civic Platform.