Rights groups on Friday condemned a "crackdown" by Saudi authorities who have arrested dozens of people, including prominent clerics, in the ultra-conservative Muslim kingdom.

Amnesty International said the authorities had detained more than 20 religious figures, writers, journalists, academics and activists in the past week.

"In recent years, we cannot recall a week in which so many prominent Saudi Arabian figures have been targeted in such a short space of time," Amnesty's Samah Hadid said.

The organisation said the rights situation in the Gulf state had "deteriorated markedly" since Prince Mohammed bin Salman took over as crown prince and heir to the throne on 21 June. 

Human Rights Watch also suggested the arrests could be connected to the prince's efforts to consolidate power.

Since 2014, Saudi Arabia has tried nearly all peaceful dissidents in its terrorism tribunal, the Specialised Criminal Court, the New York-based rights watchdog said.

"Outlandish sentences against peaceful activists and dissidents demonstrate Saudi Arabia's complete intolerance toward citizens who speak out for human rights and reform," said HRW's Middle East director Sarah Leah Whitson.

According to activists, those arrested over the past week included clerics Salman al-Awdah and Awad al-Qarni.

The preachers, who have millions of followers on social media, were among Saudi clerics who opposed the presence of US troops in the kingdom during the 1991 Gulf War over Kuwait.

They have both been accused of links to the Muslim Brotherhood, which Saudi Arabia has blacklisted as a "terror group".

Before his arrest, Awdah had welcomed the first contact between Prince Mohammed and Qatar's emir, Sheikh Tamim bin Hamad Al-Thani, after a three-month boycott of Doha by a Saudi-led bloc.

Saudi Arabia, the United Arab Emirates, Bahrain and Egypt severed all ties on June 5 and imposed economic sanctions on Qatar, accusing it of links to extremist groups and the Muslim Brotherhood, charges denied by Doha.

The Saudi authorities have not commented on the latest arrests of activists.

But at the start of the week, the attorney general warned that any attack on "national unity" or the "image of the state" amounted to a "terrorist crime".