Thousands of people joined anti-violence marches across Poland after it emerged a city mayor stabbed on Sunday evening had died of his injuries.

Pawel Adamowicz, mayor of Gdansk, died early on Monday afternoon.

He was attacked on stage during the finale of an event organised by the Great Orchestra of Christmas Charity.

Poland's health minister Lukasz Szumowski lamented to local press he could not be saved, "we couldn't win," he said.

The country's interior minister, Joachim Brudzinski, said the attack was “an act of inexplicable barbarity.”

Polish authorities identified the attacker as a 27-year-old named Stefan. His full name has been withheld.

Television footage showed the assailant shouting that he was imprisoned, despite being innocent and purportedly tortured by the former government of the Civic Platform, to which the mayor previously belonged.

The mayor, who had held the position since 1998, was seen as progressive has and shown support for LGBT rights and tolerance for minorities.

Politicians across Poland have expressed their condemnation over the stabbing. Polish President Andrzej Duda will meet with political party leaders later on Monday to organise a march against violence and hatred following the attack.

People march against violence and hatred in Warsaw.

Look down on the march in Warsaw

Anti-violence placards are held at the march in Warsaw.

European Council President Donald Tusk attends a march against violence