The centrist mayor of the Polish port city of Gdansk died on Monday, a day after a man stabbed him in the heart in front of hundreds of people at a charity event."Despite all our efforts, we failed to save him," Doctor Tomasz Stefaniak, director of Gdansk University hospital, told Polish media.Anti-violence rallies are being planned nationwide after Sunday's attack on Mayor Pawel Adamowicz, known for his liberal views and opposition to Poland's governing right-wing Law and Justice (PiS) party.European Council President Donald Tusk, tweeted that Adamowicz, "a man of Solidarity and freedom, a European, my good friend, has been murdered. May he rest in peace." Tusk, a fellow Pole born in Gdansk, was the late mayor's political ally."Goodbye Pawel, we'll remember you," Lech Walesa, Poland's legendary anti-communist leader and another native of Gdansk, said on Twitter.And Mayor of London Sadiq Khan also offered his condolences via Twitter. It was a "devastating loss" for all "who value public service, progressive policy and open, accessible to all democracy," he wrote.The 53-year-old Adamowicz had been in a critical condition with a serious wound to the heart and cuts to his diaphragm and abdominal organs, said the surgeons who operated on him for five hours.Video footage showed the attacker, who police said was wearing a press badge, bursting on to the stage and launching himself at Adamowicz, as the mayor stood with others waving sparklers towards the end of Sunday evening's fundraiser.After knifing the mayor several times, the man turned to the crowd with his arms raised triumphantly before being tackled to the ground by security guards and arrested.'Atmosphere of aggression' Paramedics resuscitated Adamowicz at the scene before rushing him to the hospital in Gdansk.Gdansk residents had flooded blood donation centres earlier on Monday following news that Adamowicz had received 15 litres (32 pints) of blood and required more of the rare O Rh- type."There is an atmosphere of aggression in Poland," a blood donor who identified himself only as Zygmunt told AFP, reflecting on the broader context of the attack.Prosecutors said they would be pressing murder charges against the detained 27-year-old male suspect. But he would undergo a psychological assessment due to "doubts about his sanity", they added.In a video recording of the attack posted on YouTube, the suspect was seen seizing the microphone and claiming he had been wrongly jailed by the previous centrist government of the Civic Platform (PO) party and tortured."That's why Adamowicz dies," he said.One witness told broadcaster TVN that the man appeared "happy with what he had done".Mayor for two decadesAdamowicz had been mayor of Gdansk for two decades. The Baltic coastal city with a population of around half a million people was the cradle of Poland's anti-communist Solidarity movement in the 1980s.The opposition Civic Platform, the arch-rival of the governing PiS, backed his re-election in the 2018 municipal polls.Since winning power nationally in 2015, the PiS has systematically blamed PO politicians for an array of issues, ranging from VAT fraud to the 2010 crash of a Polish presidential jet in Russia that claimed the life of then president Lech Kaczynski, the identical twin brother of PiS leader Jaroslaw Kaczynski.Sunday's fundraising event was part of a popular annual nationwide drive to purchase medical equipment for children and featured a colourful stage set-up including lights, smoke and pyrotechnics.Security breachThe attacker had previously been sentenced to more than five years in prison for four armed attacks on banks in Gdansk, justice officials said.Police were investigating how the attacker had been able to breach security to reach the podium, local police spokesperson Joanna Kowalik-Kosinska told reporters."We know that he used an identifier with the inscription 'Press'," she said."Now we have to establish how it was obtained, was the accreditation in his name and was he really entitled to be there at that time?"This type of attack is rare in Poland. A similar incident occurred in 2010 when an assailant gunned down an aide at a regional PiS office before stabbing another PiS employee, who survived.