KUWAIT — Kuwait arrested an opposition activist on suspicion of insulting Saudi Arabia on his Twitter account, the state news agency KUNA reported, extending a crackdown on social media.

Tariq al-Mutairi, head of the liberal Civil Democratic Movement (CDM), was detained on his way home early on Wednesday by men in plainclothes believed to be from the state security service, the alaan.cc news website said, quoting another activist who was in the same car.

KUNA, quoting an Interior Ministry statement, said Mutairi was arrested for “publishing a number of tweets insulting the sister Kingdom of Saudi Arabia.” The content of the posts was not clear. Kuwaiti law bans insults of friendly states.

CDM condemned the overnight arrest as a “kidnapping” that aimed to intimidate the group. “What the security elements have done by kidnapping the head of the movement in such a way is clearly intended to send a message of terror in the hearts of the young men and women of the movement,” a CDM statement said.