A prominent Chinese human rights lawyer who had been missing for almost a month was held in police custody and "confessed" to handing over state secrets, official media said.

Jiang Tianyong took on numerous high-profile cases, including those of Falun Gong practitioners, Tibetan protesters and victims of the 2008 contaminated milk powder scandal before being disbarred for his activism in 2009, according to campaign group Amnesty International.

He disappeared on November 21 en route to Beijing from Changsha, the capital of the central province of Hunan, where he had gone to inquire about the situation of a detained human rights lawyer.

On Friday a Communist Party newspaper reported that he had been arrested after using another person's documents to board the train and was held in administrative detention for nine days.

"Investigations showed... that he was in possession of documents containing state secrets and was in contact with foreign structures, organisations and persons, and illegally handed over state secrets abroad," the website of Fazhi Ribao (Legal Daily) said, citing police sources.

The newspaper reported that since December 1, Jiang had been subject to a "criminal coercive measure" -- a vague term which indicates a restriction on freedom of movement.

"Jiang Tianyong confessed the relevant crimes," the daily added.

Forced confessions are common in China, where courts are overseen by the ruling Communist Party.

UN Special Rapporteur on extreme poverty and human rights Philip Alston met Jiang on a visit to China in August and said last week he feared the disappearance was in part a retaliation against the lawyer's assistance to UN experts.

President Xi Jinping has overseen a tightening of controls on civil society since assuming power in 2012, closing avenues for legal activism that had opened up in recent years.

While the government initially targeted political activists and human rights campaigners, it has increasingly turned its attention to the lawyers who represent them.

In the most striking example, authorities detained more than 200 people last year during the so-called "709 crackdown" -- named after the July 9th date of the roundup -- including lawyers who took on civil rights cases considered sensitive by the ruling party.