WARSAW, Poland — The popular liberal mayor of the Polish port city of Gdansk died on Monday after he was stabbed during a charity event the previous evening by an ex-convict who stormed onstage and said it was revenge against the country’s main opposition party. Pawel Adamowicz, 53, died as a result of wounds to the heart and abdomen in spite of efforts to save him that involved a five-hour operation and blood transfusions, Health Minister Lukasz Szumowski said. “The fight for his life has been lost,” Szumowski said. The assassination of Adamowicz, a six-term mayor who often mingled freely with citizens of his city, sent Poland into shock. Even before his death was announced, marches against violence were being planned to take place across Poland in the evening.