WARSAW (JTA) — The mayor of Gdansk, Poland, who the American Jewish Committee called “a longtime friend of the Jewish community,” died a day after he was stabbed at a charity concert.

Paweł Adamowicz, a lawyer and social activist who has served as mayor since 1998, died Monday. He was 53.

On Sunday, Adamowicz took part in a charity concert organized as part of the Great Orchestra of Christmas Charity Foundation run by Polish journalist and social activist Jerzy Owsiak, who for years has been collecting funds for medical equipment for Polish hospitals. The perpetrator ran on the stage and stabbed Adamowicz several times with a knife.

The assailant had been arrested for several bank robberies and spent five years in prison. The prosecutor’s office said it suspects that he may be mentally ill.

Adamowicz opposed the extreme-right marches organized by the National Radical Camp, or ONR, in Gdansk. After one such march, in April, he personally organized an anti-fascist march.

“He was a true friend & ally in countering xenophobia. We must confront hatred in the public sphere,” AJC Central Europe tweeted.

Adamowicz also criticized last year’s Polish law on Holocaust speech, calling it “idiotic and evil.” The law outlawed rhetoric in which Poland is blamed for Nazi crimes.