Thousands of people witnessed Sunday's attack on 53-year-old Pawel Adamowicz, who was rushed to hospital, where he was operated for five hours before succumbing to his injuries. He was stabbed to the heart and to the stomach.

The frantic attack took place during the final of a charity event at the Children's Hospital, known as the Grand Christmas Orchestra, while Adamowicz thanked all those who had helped raise funds to buy medical equipment.

The suspect, 27, who was arrested, is a resident of Gdansk who was found guilty of bank robbery and spent many years in prison. Gdansk City press officer Dariusz Wołodźko said the suspect had come on the scene shouting and accused the mayor and his party of his conviction and imprisonment.

Born in Gdańsk and raised in Gdansk, Adamowicz has long been part of the public life of the city. He entered local political life in 1990 as a city councilor before becoming mayor of Gdansk, a position he held for more than 20 years.

He was known as a progressive voice in a country ruled by a populist government, resolutely right wing. He was a strong supporter of LGBT rights, immigrants and minority groups.

Silent demonstrations in solidarity with the murdered mayor and against the toxic nature of rhetoric in Polish politics are expected to take place Monday night throughout Poland.

The security personnel keep the suspect after he attacks Adamowicz.
Polish Minister of the Interior Joachim Brudzinski, describes the attack in a tweet as an "inexplicable act of barbarism".

The President of the European Council, Donald Tusk, had posted a message of support on Facebook: "Pray for Mayor Adamowicz, Pawel, we are with you."

The World Jewish Congress said that Adamowicz was an important spokesperson against right-wing extremism.

"He was a true friend of the Jewish community, speaking loudly against anti-Semitism in Poland," said CJM President Ronald S. Lauder in a statement. "It was an attack not only against the mayor, but against the very value of tolerance."

Photos shared on social media showed people lining up to give Adamowicz blood on Sunday. Hundreds of people responded to Facebook's call published by the Gdansk Regional Center on Negative Blood Groups, O RhD Group, or Universal Donors.

"About the attack of the mayor of the city of Gdańsk and the need to protect the blood, we ask you to give this valuable medicine," said the center in his message.

Moments before the attack, Adamowicz shared a photo on Instagram showing a crowd of people holding white lights at the "Lights to Heaven" fundraiser hosted by the Grand Orchester de la charité de Noël. Hundreds of people have since commented on the post with messages of remembrance.

"In my heart, Gdańsk is in first place, I want a modern, fair, friendly and open Gdańsk – Gdańsk, where everyone lives, works, develops and raises children," Adamowicz wrote on his Facebook account.

The head of the charity, Jerzy Owsiak, resigned from his post as a result of the attack.

"Let us be poles who love Polish friends, let us be Polish people who do not do violence," said Owsiak at a press conference in Warsaw. "To say that this act of violence took place in my hometown … I'm shaken."

Antonia Mortensen and Eliza Mackintosh of CNN contributed to this report.