The letter called on Xi to release all lawyers and others it said were unlawfully detained, and ensure that the detainees can pick the lawyers who represent them and are provided medical treatment and visitation rights.
Today marked the first anniversary of a crackdown in China on human rights lawyers and activists in which more than 200 were detained or questioned. Around two dozen are still detained, including several who could face life imprisonment after being charged with subverting state power.
The letter was signed by more than a dozen groups including the Hong Kong-based China Human Rights Lawyers Concern Group, the Amsterdam Bar Association, the Australian branch of the International Association of People's Lawyers and the International Commission of Jurists. It was posted on the Facebook page of the Hong Kong group.
Separately, the American Bar Association, or ABA, said in a statement that it has named a Chinese human rights lawyer, Wang Yu, recipient of a newly created "ABA International Human Rights Award."
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The ministries of justice and public security did not immediately respond to faxed requests for comment. State media have accused the lawyers and activists of disrupting public order.
Rights groups say the lawyers are being targeted for waging social media campaigns and organizing protests outside courthouses to draw attention to cases involving hot-button social issues.
It reissued an earlier European Union statement calling for China to act with greater transparency and observe due process in handling these cases. It said that the embassy's frequent attempts to "gain clarity on these issues" through diplomatic channels have remained unanswered.
Earlier in the week, UN Secretary-General Ban Ki-moon on one of his last visits to Beijing as UN leader told China's leaders that a flourishing civil society and free media are key to the country's development.