WARSAW—More than 100,000 Poles demonstrated on Sunday in support of European Union membership after a court ruling that parts of EU law are incompatible with the constitution raised concerns the country could eventually leave the bloc. Politicians across Europe voiced dismay at the ruling by Poland’s Constitutional Tribunal on Thursday, which they saw as undercutting the legal pillar on which the 27-nation EU stands. According to the organizers, protests took place in over 100 towns and cities across Poland and several cities abroad, with 80,000–100,000 people gathering in the capital Warsaw alone, waving Polish and EU flags and shouting “We are staying.” Donald Tusk, a former head of the European Council and now leader of the main opposition party Civic Platform, said the ruling Law and Justice (PiS) party’s policies were jeopardizing Poland’s future in Europe. “We know why they want to leave [the EU] … so that they can …