New Delhi, Aug. 2 -- Police in Macao detained a man believed to be a prominent pro-democracy advocate on suspicion of colluding with outside forces in breach of the national security law, in what is the first publicly known arrest under the legislation.

Local media outlets reported former pro-democracy lawmaker Au Kam San was the suspect.

A police statement Thursday said a 68-year-old Macao resident with the surname Au allegedly colluded with an anti-China group outside of the city and provided false and seditious materials for public display online or abroad.

The man, whose full name was not disclosed in the statement, also was accused of maintaining long-term contact with multiple anti-China entities outside of Macao and repeatedly p...