RIYADH (Reuters) - Saudi Arabia has detained influential religious scholar Safar al-Hawali and three of his sons, activists said on Thursday, widening an apparent crackdown against clerics, intellectuals and rights campaigners.

London-based Saudi rights group ALQST said the arrests happened after Hawali published a book critical of the Saudi royal family. ALQST’s Yahya Assiri told Reuters the arrests took place on Wednesday.

Another rights watchdog reported Hawali’s arrest, citing sources close to him.

Authorities did not immediately respond to requests for comment. Riyadh says it does not have political prisoners, but senior officials have said monitoring of activists is needed to maintain social stability.

Crown Prince Mohammed bin Salman has received praise for economic and social reforms hailed as proof of a new progressive trend, but there has been unease over the detention of women’s rights activists and a secretive anti-corruption purge last year.

Critics say the powerful son of the king is not doing enough to liberalize politics in a country where the monarch enjoys absolute authority and that he has targeted dissidents.

Hawali rose to prominence 25 years ago as a leader of the Sahwa movement, which agitated to bring democracy to Saudi Arabia and criticized the ruling family for corruption, social liberalization and working with the West.

He was jailed in a clampdown on Islamists in the 1990s but was released after muting his criticism. Following the American invasion of Iraq in 2003, Hawali backed an anti-U.S. “jihad” but also denounced Islamist militants’ attacks on Westerners in Saudi Arabia.

The Sahwa were weakened by a mixture of repression and co-optation, but remain active.

The al-Saud family has always regarded Islamist groups as the biggest internal threat to its rule over a country in which appeals to religious sentiment cannot be lightly dismissed and an al Qaeda campaign a decade ago killed hundreds.