Au is the first person to be arrested under the city's national security law.

Authorities alleged on Thursday that the 68-year-old primary school teacher had ties to foreign groups endangering China.

"This development adds to the existing concerns about the ongoing erosion of political pluralism and freedom of speech in the Macao Special Administrative Region," said European Union spokesperson Anitta Hipper in a statement.

"The EU recalls that the respect for human rights and fundamental freedoms is a central element of the Macao Basic Law and 'one country, two systems'," set up in the former Portuguese colony.

The territory near Hong Kong and known for its casinos, has retained its own legal system since China took it back from Portugal in 1999.

The security law, which restricts political activity, was passed in 2009 but broadened in 2023.

Au, a legislator up to 2021, has campaigned on social welfare, corruption and electoral reform.