Adamowicz, 53, had been mayor of the city Gdansk for 20 years and was well known for his liberal views. He was stabbed by a former convict who rushed the stage during a charity event on Sunday.
The man screamed, “Adamowicz is dead!” as he ran on stage to stab the mayor.
Before he was arrested, television footage showed the man accusing the mayor’s former party of being responsible for his incarceration. He said he had been tortured while in prison.
The attacker was a 27-year-old man called Stefan, authorities said, who had served five-and-a-half years in prison for attempted bank robbery.
“We couldn’t win,” Health Minister Lukasz Szumowski told reporters via private broadcaster TVN. Doctors had operated on Adamowicz for five hours, the state news agency PAP said.
According to a PAP source, the attacker had been previously treated in a prison hospital for schizophrenia.
Poland has designated Adamowicz’s funeral as a national day of mourning.
Thousands of Poles took to the streets to commemorate Adamowicz, including in Gdansk and the capital city Warsaw.
European Council president Donald Tusk, who is from Gdansk and a friend of Adamowicz, addressed the mayor directly, the Guardian reported: “My dear Pawel, we are here with you today as your friends. You had to wait so long, until such a tragic moment, to see from up there just how many friends you have here in Gdansk.”
The mayor was attacked at a charity event to raise money for medical equipment in hospitals. It has raised more than a billion zlotys ($265 million) over its 27-year existence.
The charity’s head resigned following the announcement of Adamowicz’s death.
Separately, a 72-year-old man was detained after threatening in a phone call that the next in line to be killed was President Andrzej Duda.
Duda appealed for unity in the face of violence, saying many Poles were united despite their political differences by “their desire to do good.”
Reuters contributed to this article.
From The Epoch Times