A senior Saudi royal and granddaughter of the country’s founding monarch has revealed she is being held in prison and demanded that the current ruler and her cousin, Crown Prince Mohammed bin Salman, release her and provide medical care.

Princess Basmah bint Saud bin Abdulaziz al-Saud, an outspoken human rights advocate, claims she is being detained without charge in Riyadh with one of her daughters. She says neither have received explanations for their arrests, despite repeated pleas to the kingdom’s royal court, and to her uncle King Salman.

The princess, the last of King Saud’s 108 children, took to Twitter to plead for freedom and claim her health had deteriorated to the point that it was now “very critical”. She said she was being held in al-Ha’ir prison, and claimed to have done no wrong.

News of her detention surprised two senior royals who said she had not been heard from for close to a year, and was thought to be convalescing after a bout of illness. Other members of the extended family believed her to be under house arrest.

It is understood that Princess Basmah and her daughter were detained as they tried to leave Saudi Arabia for Switzerland in March last year. She claimed then to be in urgent need of medical treatment. However her private jet was not allowed to depart.

Relatives had since had fleeting conversations with the 52-year-old, with one claiming on Thursday that she had been guarded and appeared to be speaking under duress. Princess Basmah had been a frequent advocate of reform in the kingdom, and had advanced women’s rights and humanitarian rights during a brief media career, and several years in London, where she developed a business career.

She had called for Saudi Arabia to become a constitutional monarchy, a change that would have separated the position of monarch from the country’s executive branch – a fundamental change to its current status of an absolute monarchy.

She returned to the kingdom in late 2015, assuming a role of supporter of the royal family on one hand, but in-house critic on the other. She urged restraint in the Saudi-led war in Yemen and widespread reforms at home.

Human Rights Watch (HRW) said the arrest fitted a clear pattern of dissenters being ruthlessly silenced by Prince Mohammed, who has methodically consolidated power since ousting his uncle Mohammed bin Nayef nearly three years ago and giving himself a clear run to the throne.

“Under Crown Prince Mohammed bin Salman’s rampant repression of all forms of critics, including people he can extort money out of, the space for dissent has shrunk greatly,” said Rothna Begum, senior women’s rights researcher at HRW. “This is particularly the case for women, many of whom have been silenced, imprisoned, or are in exile right now.

“No one is off limits to the crown prince. He really is after everyone and women have borne the brunt of this. We are seeing things that we never saw before in Saudi. There was a time when women of a powerful background could say things about women’s rights and issues that matter. But under the crown prince, this important space has gone.”

In early March, Bin Nayef and the last remaining full brother of King Salman, Ahmad bin Abdul Aziz al-Saud were arrested in their homes and accused of plotting against the crown prince. They were the most senior royals yet to be arrested in a series of purges over the past two years that have included business leaders, senior military officers and members of the inner sanctum. Both men remain in detention. The Guardian revealed at the time that both men had been accused of discussing using a newly formed body, the allegiance council, to try to prevent Prince Mohammed from being named as king if his father dies.

The arrests have sparked fear in senior circles and across large parts of Saudi society. Critics say they offset civic reforms that have been introduced in the same time, which give women a broader role in society and more power to shape their destinies, and have rescinded decades of rigid theocratic rule.

Prince Mohammed has demanded total loyalty from family members and has shown himself willing to take on royals who were considered untouchable and tribes that were too influential to tangle with. He has been accused of ordering the killing of prominent critic Jamal Khashoggi in Istanbul in October 2018 – a gruesome assassination that battered his relations with former allies. Prince Mohammed’s father, the 84-year-old monarch, has given him unfettered powers to put the kingdom on a new economic and social footing that could wean it off a dependency on oil, attract investors, and bring the country more into line culturally with neighbouring states, such as the United Arab Emirates.

An adviser to Mohammed bin Salman was approached for comment about the allegations. He declined to comment.