BEIRUT, Lebanon — For years, she was a rare princess from Saudi Arabia who spoke her mind to the world, criticizing the kingdom’s treatment of women, calling its religious teachings “extremely dangerous” and voicing support for a constitutional monarchy.

And she got away with it — until she disappeared last March.

This week, the princess, Basmah bint Saud, a daughter of Saudi Arabia’s second king, confirmed what had long been suspected: A statement on her Twitter feed said that she was being held in a notorious prison in Saudi Arabia without charge, and that she was in urgent need of medical care.

“I was abducted without explanation together with one of my daughters and thrown into prison,” she wrote. She begged Saudi Arabia’s king and the crown prince “to release me as I have done no wrong.”

By Friday, that plea had been deleted.

The reason for her arrest was not clear, but it appeared to fit a pattern of Saudi Arabia’s government punishing prominent citizens who had publicly deviated from the government’s line.