Thousands of people joined marches across Poland to pay tribute to the mayor of Gdansk, who died on January 14 after being stabbed in the heart at a charity event.
Pawel Adamowicz died in the hospital after a long fight by doctors to save him.
Thousands marched in Gdansk, the Baltic city Adamowicz ran for two decades.
European Council President Donald Tusk flew in to his hometown to attend the march in memory of his friend and former political ally.
Thousands more took to the streets of the capital, Warsaw.
Adamowicz had been in critical condition for hours after the attack and died on January 14, health officials said.
"Despite all our efforts, we failed to save him," said Dr. Tomasz Stefaniak, Gdansk University hospital director.
In the January 13 attack, Adamowicz suffered a serious wound to the heart and cuts to abdominal organs and was operated on for five hours.
The 27-year-old suspect, who has a criminal record, was detained at the scene. His full identity has thus far been withheld.
A video showed the man seizing a microphone at the Light to Heaven charity event and claiming he had been wrongly jailed by the previous government of the center-left Civic Platform party and tortured.
"That's why Adamowicz dies," he said, before being detained.
A liberal, Adamowicz had been mayor of the Baltic port city for two decades and the opposition Civic Platform supported his reelection last year. He was an outspoken opponent of the ruling right-wing Law and Justice Party and had been an advocate for encouraging migrants to seek refuge in Gdansk.
Polish President Andrzej Duda said he would meet with political party leaders on January 14 to organize a march against violence and hatred.
Gdansk is home to the shipyard where the Solidarity labor union led by Lech Walesa was founded in 1980 and became a force that eventually helped contribute to the fall of communism.
