Amnesty International says Saudi authorities have unleashed a "systematic and ruthless campaign of persecution" against peaceful activists to silence criticism of the state since the 2011 Arab Spring protests.

The human rights group issued a 20-page report early Friday titled "Saudi Arabia's ACPRA: How the kingdom silences its human rights activists." It focuses on legal cases levied against 11 founding members of the Saudi Civil and Political Rights Association.

Amnesty says the association's founders have been targeted in a broad crackdown on freedom of expression that intensified in recent years. Eight of them remain detained, along with other prominent rights activists.

Saudi Arabia is an absolute monarchy with no elected parliament. Saudi officials have traditionally not responded to criticism from rights groups.