European leaders are condemning the fatal stabbing of the mayor of the Polish city of Gdansk as a "shameless act" and a "senseless act of violence".

German President Frank-Walter Steinmeier wrote to Pawel Adamowicz's widow and said he was "particularly shocked that your husband was attacked during the closing rally of a donation campaign to raise funds for hospitals".

The mayor died this morning, less than a day after he was stabbed on stage at the fundraiser.

Czech Foreign Minister Tomas Petricek called Adamowicz's slaying "a great tragedy not only for Gdansk and Poland but for the whole of Europe and its citizens."

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Petricek wrote on Twitter: "It is important to make clear that, irrespective of differences in political attitudes, we all strongly condemn this shameful act."

People in Poland are gathering for solemn vigils to honour the mayor.

Some Poles held candles flickering in glass holders as they gathered in Gdansk. There were also vigils in Warsaw and other cities across a nation shocked by the assassination.

In the Polish capital, some held a big banner reading "Stop Hate."Adamowicz was stabbed multiple times by an assailant who stormed the stage at the "Lights to Heaven" fundraiser.

The assailant shouted it was revenge against Poland's main opposition party, which Adamowicz left in 2015.

Poland's President says a national day of mourning will be observed.

President Andrzej Duda opened a news conference with a minute of silence in memory of Adamowicz.

Duda said the attack was an "evil hard to imagine" and a "blow to our community of people who want to do good".

He called Adamowicz a "truly great human being, a great politician and great resident of Gdansk".

Funeral and burial plans are pending.

- AP

Gdansk Mayor Pawel Adamowicz who was stabbed to death. Photo / AP file
Gdansk Mayor Pawel Adamowicz who was stabbed to death. Photo / AP file