Saudi authorities on Wednesday released prominent women's rights activist Loujain al-Hathloul after nearly three years in detention, her family said, as the kingdom comes under renewed US pressure over its human rights record.

Hathloul, 31, was arrested in May 2018 with about a dozen other women activists just weeks before the historic lifting of a decades-long ban on female drivers, a reform they had long campaigned for, triggering a torrent of international criticism.

The release of the activist, who is still under probation and is barred from leaving Saudi Arabia, sparked euphoria among her siblings who had launched a vigorous campaign overseas for her freedom in a major embarrassment for the kingdom's rulers.

"Loujain is at home!!!!!!!" her sister Lina al-Hathloul wrote on Twitter.

"At home after 1001 days in prison," she added, along with a picture of the smiling activist with streaks of grey hair.

US President Joe Biden, who has pledged to intensify scrutiny of Crown Prince Mohammed bin Salman's human rights record, welcomed the decision to release her, saying it was "the right thing to do".

The US State Department said she should never have been jailed.

"Promoting and advocating for women's rights and other human rights should never be criminalised," said State Department spokesman Ned Price.

French President Emmanuel Macron, who strongly called for Hathloul's release last year, welcomed the news, tweeting that he "shared the relief of her family". 

In late December, a Saudi court handed Hathloul a prison term of five years and eight months for terrorism-related crimes, but her family said a partially suspended sentence paved the way for her early release.

The women's rights activist was convicted of inciting regime change and seeking to disrupt public order, in what her family denounced as a "sham" trial. They also alleged she experienced sexual harassment and torture in detention, claims dismissed by the court.

The verdict, however, was a "face-saving exit strategy" for the Saudi government after it came under severe international pressure to free her, a source close to Hathloul's family told AFP at the time.

Still, the court has imposed a five-year travel ban on Hathloul, her relatives said.

"Loujain is at home, but she is not free. The fight is not over," Lina said.

- Biden's 'impact' -

Saudi authorities have not officially commented on her detention, trial or release.

"Loujain al-Hathloul's release after a harrowing ordeal in prison in Saudi Arabia -- lasting nearly three years -- is an incredible relief, but long overdue," said Amnesty International's Lynn Maalouf.

"Nothing can make up for the cruel treatment she has suffered, nor the injustice of her imprisonment."

After the kingdom largely got a free pass under previous president Donald Trump, Biden is expected to push it to free dual US-Saudi citizens, activists and royal family members, many of whom are detained without any formal charges.

"Elections matter. The arrival of a Biden administration that has put human rights and values at the top of its Saudi agenda is having an impact," Kristin Diwan, of the Arab Gulf States Institute in Washington, told AFP.

"More needs to be done before this could be seen as progress in terms of human rights. Saudi Arabia has not credibly investigated the torture allegations, and has not dismissed charges against Loujain."

After being tried in Riyadh's criminal court, Hathloul's case was transferred in November to the anti-terrorism court, which campaigners say is used to silence critical voices under the cover of fighting terrorism.

Foreign Minister Prince Faisal bin Farhan subsequently told AFP that Hathloul was accused of contacting "unfriendly" states and providing classified information, but her family said no evidence to support the allegations had been put forward.

While some women activists detained along with Hathloul have been provisionally released, many others remain imprisoned on what rights groups describe as opaque charges.

The detentions have cast a spotlight on the human rights record of the kingdom, an absolute monarchy which has also faced intense criticism over the 2018 murder of journalist Jamal Khashoggi in its Istanbul consulate.