The Chinese casino hub, which has its own legal system largely based on Portuguese law, enacted national security legislation in 2009 and widened its powers in 2023.

Macau's judicial police said a 68-year-old local man surnamed Au was arrested and handed over to public prosecutors on suspicion of "establishing connections... outside Macau to commit acts endangering national security".

Local media identified the man as Au Kam-san, a primary school teacher who became one of Macau's longest-serving pro-democracy legislators before deciding not to seek re-election in 2021.

The man allegedly provided "a large amount of false and seditious information to an anti-China group" for public exhibitions online and abroad since 2022, and "stirred up hatred" against the Macau and Beijing governments.

He is also accused of spreading false information to various groups, which allegedly disrupted the city's 2024 leadership election and caused foreign countries to take hostile action against Macau, police said in a statement, without naming the groups.

A stalwart of Macau's tiny opposition camp, Au spent years campaigning on issues such as social welfare, corruption and electoral reform.

Online news platform All About Macau reported that judicial police took away the ex-lawmaker and his wife Virginia Cheang on Wednesday.

Cheang told the outlet outside the public prosecution office on Thursday that she was listed as a witness and that she did not know why her husband was detained.

AFP was unable to reach Au for comment.

- Chill on dissent -

The former Portuguese colony reverted to Chinese rule in 1999 via a "One Country, Two Systems" framework that promised a high degree of autonomy and rights protections.

For years it was regarded by Beijing as a poster child in contrast with neighbouring Hong Kong, which often saw boisterous protests.

The high-water mark of Macau activism came in 2014 when some 200,000 people rallied to oppose granting perks to retired government officials, an event that Au helped to organise.

One pro-establishment Macau lawmaker told a newspaper in 2020 that the city was threat-free, as shown by the fact that the "national security law had never been used... in 11 years".

But when Beijing cracked down on Hong Kong after months of huge and sometimes violent pro-democracy protests in 2019, similar curbs were extended to Macau.

The casino hub expanded the scope of national security laws in May 2023, which officials said was meant to step up prevention of foreign interference.

Former top judge Sam Hou-fai became Macau's leader in December after a one-horse race.

City officials this month disqualified 12 candidates from the legislative elections set for September, saying they did not uphold Macau's mini-constitution or pledge allegiance to the city.

The dozen hopefuls include sitting lawmaker Ron Lam, who said last week that the grounds for barring him were "ridiculous".