"Spring" aims to offer a progressive alternative to the ruling right-wing Law and Order (PiS) and business-friendly Civic Platform (PO). Founder Robert Biedron was Poland's first openly gay mayor from 2014 to 2018.
Robert Biedron, who named the party "Wiosna" ("Spring"), said he wanted to challenge Poland's divisive politics and win seats from Poland's ruling right-wing Law and Justice (PiS) party at European elections in May.
"There is no room for hate, we have reached the limit," he said in front of thousands of supporters in the capital, Warsaw. "We need a spring that will renew this gloomy landscape," he added.
The youthful and charismatic 42-year-old's rise has come as older left-wing leaders have retired or fallen to corruption scandals.
His political stances, which include advocating the end of religious teachings in schools, are controversial in Poland, where the Catholic Church and coal industry are influential.
Speaking in Warsaw on Sunday, Biedron vowed to continue where Adamowicz had left off.
"We need positive energy more than ever now," he said. "We must fulfill the legacy of Pawel Adamowicz."
Poland mourns Gdansk Mayor Pawel Adamowicz
Several thousand people mourned the death of Gdansk Mayor Pawel Adamowicz after he was stabbed at a charity event. Marches took place across the country, including Poznan, Krakow and the capital Warsaw.
Image: picture-alliance/B. Zawrzel
Poland in mourning
Candles surrounded a picture of Gdansk Mayor Pawel Adamowicz as people paid tribute to him after he was killed. Adamowicz died in hospital on Monday, January 14 after being stabbed at a charity event the day before.
Image: picture-alliance/NurPhoto/K. Dobuszynski
Christmas charity
Adamowicz, 53, was attending an event organized by the Great Orchestra of Christmas Charity when a man stormed onstage and stabbed him. The Gdansk mayor was rushed to the hospital and underwent five hours of surgery in an attempt to save his life.
Image: Reuters/Agencja Gazeta/B. Banka
Accused in custody
Security personnel quickly pinned down the perpetrator before police arrested him. A spectator video showed the suspect, allegedly a 27-year-old who was released from prison last month, claiming he had been wrongly jailed and tortured by the government.
Image: Getty Images/AFP/P. Hukalo
March in Gdansk
Thousands of people marched against violence and hatred in Gdansk in the wake of Adamowicz's death. Gdansk, a port city in northern Poland, was the birthplace of Solidarity, an anti-communist trade union and social movement in the 1980s in which the late mayor took part.
Image: Reuters/Agencja Gazeta/Renata Dabrowska
Tusk in Gdansk
Donald Tusk, the president of the European Council and a former Polish Prime Minister, led a moment of silence at a gathering at the main old city square in Gdansk to mourn Adamowicz's death. "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.
Image: picture-alliance/M. Fludra
'Stop hatred'
Several thousand people also marched through the Polish capital of Warsaw after Adamowicz' death. One held a sign reading "stop the politics of hatred." Marches also took place in the cities of Krakow in the south and Poznan in central Poland.
