King Abdullah Monday decreed jail terms of up to 20 years for belonging to "terrorist groups" and fighting abroad, as it struggles to deter Islamist Saudis from becoming jihadists.
"Taking part in combat outside the kingdom, in any form" will be punished by jail terms of between three and 20 years, said the decree published by state news agency SPA.
Similar sentences will be passed on those belonging to "extremist religious and ideological groups, or those classified as terrorist organisations, domestically, regionally and internationally," it said.
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Supporting such groups, adopting their ideology or promoting them "through speech or writing" would also incur prison terms, the decree added.
Rights group Amnesty International sharply criticised the new legislation, saying it could be used to suppress peaceful political dissent because the law used an "overly vague definition of terrorism".
"The Saudi Arabian authorities are seeking legal cover to entrench their ability to crack down on peaceful dissent and silence human rights defenders," Amnesty's Said Boumedouha said in a statement.
Saudi Arabia set up specialised terrorism courts in 2011 to try dozens of nationals and foreigners accused of belonging to Al-Qaeda or being involved in a wave of bloody attacks that swept the country from 2003.
Scores of Saudis are believed to have joined Islamist extremists fighting in Syria, where Riyadh is a strong backer of the nearly three-year rebellion against the regime of President Bashar al-Assad.