Vigils were held on Monday in Gdansk, Warsaw and other cities across Poland following the assassination of popular liberal mayor Pawel Adamowicz.
Many carried candles in glass jars and some used phones to produce flickers of light, an apparent reference to the Lights to Heaven event where the politician was slain.
Donald Tusk, a founder of Civic Platform who was prime minister when the attacker was imprisoned and who is now president of the European Council, joined mourners in Gdansk, also his home town.
“I want to promise you today, dear Pawel, that for you and for all of us, we will defend our Gdansk, our Poland and our Europe from hatred and contempt,” Tusk told the crowd.
The city flag was lowered to half-mast and masses were held in Gdansk, Warsaw and elsewhere.
A number of social media users shared photographs of Polish well-wishers who took to the streets to pay tribute to the fallen leader.
President Andrzej Duda, who is aligned with the ruling party, opened a news conference with a minute of silence and said a day of national mourning will be observed when Adamowicz’s family holds his funeral. He is survived by his wife and two daughters.
Duda called him a “truly great human being, a great politician and great resident of Gdansk”, and denounced the attack as an act of unimaginable evil.
He tweeted: “Mr. President Paweł Adamowicz has gone. I combine regret and prayer with his closest friends and all men of good will. Please Lord God for support for the deceased family. Hostility and violence brought pain.”
Ruling party leader Jaroslaw Kaczynski, Poland’s most powerful politician, expressed his “great pain” and solidarity with Adamowicz’s family.
Aleksandra Nabożny, who came with her father to pay her respects at a candlelit vigil by the statue of Neptune, said: “We are very sad, and deeply, deeply hurt by what has happened.
“We used to see him walking to work in his suit and with his briefcase, stopping and talking to people, wanting to be kind to everyone. He and the city were a perfect match.”
The 53-year-old died from the wounds inflicted by a 27-year-old man, who was later arrested, stormed onstage on Sunday evening while he was addressing an audience during the Lights to Heaven, the finale of a nationwide fundraiser for sick children.
Three people have been arrested after calling for more killings following the death of the mayor of Gdansk, the interior ministry confirmed on Tuesday.
Police arrested the three on Monday, Polish Conservative politician Joachim Brudzinski said.
He described those arrested as internet trolls and “unbalanced”, though it is not clear whose murder they were calling for.
Just before he was attacked, Adamowicz had expressed gratitude to the “generous” crowd, adding: “This is a wonderful time of sharing good things. You are dear. Gdansk is the most wonderful city in the world. Thank you!”
The assailant rushed up and stabbed him three times, then grabbed a microphone to tell the audience that the attack was an act of revenge against a political party Adamowicz had belonged to.
The mayor was taken to hospital where doctors struggled to save him, but a five-hour operation and blood transfusions were not enough.
“We couldn’t win,” said Poland’s health minister, Łukasz Szumowski.