Mr. Ai was the most prominent of hundreds of people detained since China intensified a broad crackdown on critics of the government in February, when anonymous calls for mass protests modeled after the revolutions in the Arab world percolated on the Chinese Internet.
China’s move to douse any flicker of dissent was the harshest in years outside of the restive ethnic regions in the far west, and the vast majority of those detained in the crackdown were, like Mr. Ai, held in secret locations for weeks with no legal justification.