Saudi Arabia has arrested at least eight writers and bloggers, including two US citizens, in an apparent crackdown on supporters of detained women activists whose trial has drawn Western condemnation, campaigners said.

The 11 women on trial had campaigned for the right to drive and an end to the kingdom's male guardianship system. Their case has intensified criticism of Riyadh's rights record.

News of the arrests came the morning after US lawmakers voted to end military support for a Saudi-led war in neighbouring Yemen, which has triggered what the UN describes as the world's worst humanitarian crisis.

It also marks the first major crackdown since the brutal murder of journalist Jamal Khashoggi inside the Saudi consulate in Istanbul last October, which sparked unprecedented international scrutiny of the kingdom's human rights record.

Campaigners identified the US-Saudi dual nationals as writer and doctor Bader al-Ibrahim and Salah al-Haidar, the son of Aziza al-Yousef - a high-profile activist who was temporarily released last week but remains on trial along with other women campaigners.

The latest round up started on Thursday, a day after the 11 women - some of whom accused interrogators of torture and sexual abuse in detention - returned to court to face charges that include contact with foreign media, diplomats and human rights groups.

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But one of them, university lecturer Anas al-Mazrou, was detained last month after he expressed solidarity with the detained women during a panel discussion at the Riyadh book fair, multiple activists said.

London-based rights group ALQST said all eight were "writers and social media bloggers previously engaged in public discourse on reforms" and that the crackdown was linked to their support of women activists on trial.

Prisoners of Conscience, a Saudi group that tracks political prisoners, put the number of people arrested at 10.

"Saudi authorities seem to be hellbent on silencing anyone who dares to speak up or even voice their opinions privately and publicly," Amnesty International's Middle East campaigns director Samah Hadid told AFP.

"This goes beyond just attacking activists and now seems to target different segments of society. This is a dangerous pattern."

Pressure to stay silent

There was no immediate comment from Saudi authorities or the US embassy in Riyadh.

"What is disturbing about new Saudi arrests is that waves of arrests keep moving from most-known to successively lesser knowns," Saudi-American activist Nora Abdulkarim said on Twitter.

"Another confusing aspect is timing, leaves one asking: 'why now?'"

Riyadh has faced pressure from major Western powers to release the women on trial, most of whom were detained last summer in a wide-ranging crackdown against activists just before the historic lifting of a decades-long ban on female motorists.

Three of them - activist Aziza al-Yousef, blogger Eman al-Nafjan and preacher Rokaya al-Mohareb - were granted temporary release last week.

Before their provisional release, the three freed women and their bail guarantors were made to sign a pledge that they will stay away from the media, according to multiple people with access to the trial.

The siblings of one of the prominent detainees, activist Loujain al-Hathloul, this week said they were being pressured by people close to the Saudi state to stay silent over her treatment in detention.

"A reminder: we are not going to stay silent"

People close to the Saudi establishment have warned that public criticism by family members could prolong their detention.

"A reminder: we are not going to stay silent," Loujain's sister Alia al-Hathloul tweeted on Friday.

At an emotionally charged hearing last week, some women broke down as they accused interrogators of subjecting them to electric shocks, flogging and groping in detention, two people with access to the trial told AFP.

A Saudi prosecutor roundly rejected the accusations in the latest court hearing on Wednesday, witnesses said, reiterating the government's stance.

The next hearing in the trial, which foreign media and diplomats are barred from attending, is scheduled for April 17, activists say.

Fears for their safety

Responding to reports of the arrests, Amnesty International's Middle East Director of Research Lynn Maalouf said Saudi Arabia is setting a dangerous precedent.

"Those arrested include Salah al-Haidar, the son of human rights activist Aziza Al-Yousef, who was temporarily released just a few days ago after more than 10 months of a terrible ordeal. Others include Abdullah al-Duhailan, a journalist, novelist and advocate for Palestinian rights and Fahad Abalkhail, who has supported the Women to Drive Campaign."

"We are deeply concerned for the wellbeing of those detained since yesterday."

"In their continuing crackdown, it is no coincidence that the Saudi Arabian authorities are shamelessly targeting those citizens who are part and parcel of the society's vibrant intellectual, artistic, activist landscape. By targeting them, they are signaling to their entire people that there will be zero tolerance of any form of criticism, let alone questioning, of the state's authoritarian practices.

"We are deeply concerned for the wellbeing of those detained since yesterday. The Saudi Arabian government must reveal their whereabouts, ensure they are protected from torture and other ill-treatment, and allow them access to lawyers, as well as contact with their families.

"We call on the authorities to immediately disclose the charges brought against those detained, and if they are detained for their peaceful exercise of their right to freedom of expression, then they should be released immediately and unconditionally."