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."
"In connection with the attack on the Mayor of the City of Gdańsk and the need to make sure that blood is protected, we are asking you to give this precious medicine," the center said in the post.
Just moments before the attack, Adamowicz shared a photo on Instagram showing a crowd of people holding white lights during "Lights to Heaven" fundraiser, organized by the Great Orchestra of Christmas Charity. Hundreds of people have since commented on the post with messages of remembrance.
"In my heart, Gdańsk occupies the first place. I want a modern, fair, friendly and open Gdańsk. Gdańsk, where everyone lives well, works, develops and brings up children," Adamowicz wrote of himself on his Facebook account.
The head of the charity, Jerzy Owsiak, resigned from his post in the wake of the attack.
"Let's be Poles who love one another. Let's be Poles who are friends to one another. Let's be Poles who have no violence towards one another," said Owsiak during a press conference in Warsaw. "To think that this act of violence took place in my hometown... I am shaken."
Silent demonstrations in solidarity with the slain mayor are expected to take place across Poland Monday evening.