* Exiled opposition has called for demonstrations on Friday
Saudi Arabia has urged its people to report subversive comments spotted on social media via a phone app, a move denounced by a human rights watchdog as "Orwellian".
The appeal, announced on a Twitter account run by the interior ministry late on Tuesday, coincides with an apparent crackdown on potential government critics and a call by exiled opposition figures for demonstrations.
"When you notice any account on social networks publishing terrorist or extremist ideas, please report it immediately via the application #We're_all_security," it said, referring to a mobile phone app launched last year to enable civilians to report traffic violations and burglaries.
Hours later, the public prosecutor tweeted a section of the kingdom's terrorism law, which states: "Endangering national unity, obstructing the Basic Law of governance or some of its articles, and harming the state's reputation or status are terrorist crimes."
Exiled Saudi critics have called for demonstrations on Friday to galvanise opposition to the royal family, while clerics, intellectuals and activists, including prominent Islamist cleric Sheikh Salman al-Awdah, have been detained this week, activists say.
Activists circulated lists of people detained on social media showing the number had risen to around 30 on Wednesday, including some with no clear links to Islamist activity or obvious history of opposition.
Protests are banned in Saudi Arabia, as are political parties. Unions are illegal, the press is controlled and criticism of the royal family can lead to prison.
Riyadh says it does not have political prisoners, but senior officials have said monitoring activists is needed to maintain social stability.
Human Rights Watch, a New York-based watchdog, condemned the government dragnet, saying it called into question the authorities' commitment to free speech and the rule of law.
"Saudi Arabia is reaching a new level of Orwellian reality when it goes beyond security services' repression and outsources monitoring of citizens' online comments to other citizens," said Middle East director Sarah Leah Whitson, referring to English writer George Orwell's dystopian novel Nineteen Eighty-Four.
"Saudi Arabia's new leadership is quickly showing it has no tolerance for critical thought or speech and is marshalling Saudi society to enforce red lines by spying on itself."
The government has not clearly acknowledged this week's detentions or responded to requests for comment.
But state news agency SPA said on Tuesday that the authorities had uncovered "intelligence activities for the benefit of foreign parties" by a group of people it did not identify.
A Saudi security source told Reuters the suspects were accused of "espionage activities and having contacts with external entities, including the Muslim Brotherhood", which Riyadh has classified as a terrorist organisation.
The government toughened its stance on dissent following the Arab Spring in 2011 after it averted unrest by offering billions of dollars in handouts and state spending.
But the Brotherhood has gained power elsewhere in the region.
The Brotherhood-inspired Sahwa movement in the 1990s agitated to bring democracy to Saudi Arabia and criticised the ruling family for corruption, social liberalisation and working with the West, including allowing US troops into the kingdom during the 1991 Iraq war.
The Sahwa were largely undermined 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.
Saudi Arabia, the UAE, Bahrain and Egypt cut diplomatic and transport links with Qatar in June over its alleged support for militants, a charge that Doha denies.