The mayor of the Polish city of Gdansk, a well-known critic of his country's rightwing government, died on Monday, one day after he was stabbed by a man who rushed on stage during an open-air charity event.

Thousands of people witnessed Sunday's attack on Pawel Adamowicz, 53, who was rushed to hospital where he underwent a five-hour long operation before succumbing to his injuries. He was stabbed in the heart and stomach.

The brazen attack took place during the finale of a children's hospital charity event, known as the Great Orchestra of Christmas, when Adamowicz was thanking everyone who had helped raise funds for medical equipment.

The 27-year-old suspect, who was arrested, is a Gdansk resident with convictions for bank robbery and had spent time in prison. Gdansk city press officer Dariusz Wołodźko said the suspect came onstage shouting, and blamed the mayor and his party for his conviction and imprisonment.

Born and raised in Gdansk, Adamowicz had long been a fixture of public life in the city. He entered local politics in 1990 as a local councilor before serving as Gdansk's mayor, a position he held for more than 20 years.

He was known as a progressive voice in a country ruled by a populist, staunchly rightwing government. He was a staunch supporter of LGBT rights, immigrants, and minority groups.

Silent demonstrations in solidarity with the slain mayor, and against the toxic nature of rhetoric in Polish politics, are expected to take place across Poland Monday evening.

Poland's Interior Minister, Joachim Brudzinski, described the attack in a tweet as an "act of inexplicable barbarity."

European Council President Donald Tusk had posted a message of support on Facebook, saying, "let's pray for Mayor Adamowicz. Pawel, we are with you."

Photos shared on social media showed people lining up to give blood for Adamowicz on Sunday. Hundreds of people responded to Gdansk's regional center's Facebook post appealing for blood group O RhD negative, or universal donors. (CNN)

• Security personnel hold down the suspect after he attacked Adamowicz.

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