BANGKOK (AP) — Police in Thailand have arrested a political activist accused of threatening national security by posting a comment on Facebook about how royal families in other nations lost power.

The arrest Monday night of Karn Pongpraphapan came after an unprecedented amount of social media commentary last week that implicitly criticized Thailand's monarchy by complaining about congestion caused by police blocking traffic for the motorcades of royal family members.

Criticism of the monarchy is taboo, and circulating material deemed insulting to it is punishable by 3-15 years' imprisonment.

The blocking of traffic for royal vehicles has long been the subject of private grumbling, but recent road congestion unleashed a torrent of tweets with the hashtag #royalmotorcade, some veering into sharper comments. A police statement Tuesday related the arrest to "inappropriate hashtags."