BRUSSELS—The European Union called for the immediate release Wednesday of the 53 former lawmakers and pro-democracy activists who have been arrested in Hong Kong after being accused of violating a new security law. The mass arrests, including of former lawmakers, were the largest crackdown on Hong Kong’s democracy movement since the law was imposed by China last June to quell dissent in the semi-autonomous territory. EU spokesman Peter Stano told a press conference that the arrests send the “signal that political pluralism is no longer tolerated in Hong Kong.” He added that the security law is being used “to crush dissent and stifle the exercise of human rights and political freedoms.” Stano did not exclude the possibility of sanctions against China, saying that EU authorities and member nations are currently “thinking about the best measures to take to react.” Stano said the issue could be discussed during a meeting of …