Dubai - Saudi Arabia has detained thousands of people for years without trial, Human Rights Watch said on Sunday, slamming the country's powerful crown prince for the "arbitrary detentions".

Official data from the interior ministry, analysed by HRW, showed that authorities had detained 2305 people for more than six months without referring them to court.

More than 1870 had been held for more than a year and 251 for more than three years with their cases still "under investigation", HRW said, citing the ministry database.

One person has been held for over a decade in what HRW said was a case of "documented arbitrary detention".

The ultraconservative kingdom, an absolute monarchy, has introduced a string of reform in past months, spearheaded by the country's unchallenged Crown Prince Mohammed bin Salman, dubbed "MBS", who was appointed heir to the throne in June 2017.

Yet arbitrary detention appears to have "increased dramatically in recent years", according to HRW. The group urged authorities to "stop holding people arbitrarily".

"If Saudi authorities can hold a detainee for months on end with no charges, it's clear that the Saudi criminal justice system remains broken and unjust, and it only seems to be getting worse," said Sarah Leah Whitson, Middle East director for the New York-based rights group.

"It seems that MBS's Vision 2030 plan better describes the length of detentions without charge than an aspirational time horizon for reforms."

Mohammed bin Salman is the architect of his country's "Vision 2030" plan, a sweeping reform project aimed at weaning Saudi Arabia off of its dependence on oil and modernising one of the most restrictive countries in the world.

The project includes plans to privatise part of oil giant Aramco and boost the role of women in the workforce.

Women will be permitted to drive in Saudi Arabia from June.

The kingdom has one of the highest execution rates in the world.