As many as 100,000 people thronged a square at Warsaw’s historic old town on Sunday evening after former European Council president and opposition leader Donald Tusk urged his compatriots to protest the ruling that rejected a pillar of EU law. The judgment threatens to sink billions of euros in aid and has sparked concerns the biggest eastern member might one day end up leaving the bloc.

Tusk told the crowd said it’s time to “raise alarm because the ruling party decided to take Poland out of the EU” and is trampling democratic norms. “We need to show that we are ready to protest, but also to win,” he said.

Read More: Polish Ruling Puts EU in a Multibillion-Euro Bind

Prime Minister Mateusz Morawiecki, who had brought the contested case to the court, has insisted that Poland will remain in the EU despite the standoff. The ruling nationalists have spared with Brussels over everything from changes to courts, media freedom and LGBTQ rights since they took power in 2015.

Pro-EU protests took place in some 120 towns and cities across Poland on Sunday, according to Gazeta Wyborcza daily.