A Chinese journalist who has been missing for several days has been detained because he is "implicated in a case", his lawyer Sunday cited authorities as saying.

Jia Jia disappeared Tuesday shortly after going through customs at Beijing international airport while preparing to board a flight to Hong Kong where he was to attend an academic conference, according to a close friend cited by Amnesty International.

"Because of his implication in a case, Jia Jia was detained at Beijing airport terminal on March 15," said lawyer Yan Xin on his WeChat account, adding that the information came from the city's Public Security Bureau.

No details were given about the nature of the case.

Amnesty and the Committee to Protect Journalists linked the journalist's disappearance to the publication this month on the Chinese news website Wujie News of an open letter calling for the resignation of Chinese President Xi Jinping.

The letter, which was rapidly removed, was signed "Loyal Communist Party members", but little else is known about its authorship.

"His going missing is most likely related to the publishing of the letter and perhaps the authorities' implication of his involvement or knowledge of the letter," Amnesty China researcher William Nee told AFP earlier.

However, Jia's lawyer Yan has said his disappearance may not be connected to the letter.

Since Xi came to power the Chinese Communist Party has increased pressure on civil society, arresting or questioning more than 200 lawyers and human rights activists.

China is currently in the spotlight over the disappearance last year of five Hong Kong booksellers who reappeared on the mainland, as well as the use of televised confessions from suspects.

The five booksellers were from Hong Kong's Mighty Current publishing house, known for its salacious titles critical of Beijing.

Hong Kong student leader Wong to set up political party
Hong Kong (AFP) March 20, 2016 - The teenage face of Hong Kong's pro-democracy movement announced on Sunday he will head a new political party to be set up to contest legislative elections in September.

Joshua Wong, 19, was at the forefront of mass rallies which brought parts of the semi-autonomous Chinese city to a standstill in 2014 as residents called for fully free elections for future leaders.

The 79-day protest which blocked several major roads failed to win any concessions from the Hong Kong or Chinese governments.

Wong's announcement came at a time when more radical young activists are gaining a foothold in mainstream politics, with one student leader who calls for independence from China taking tens of thousands of votes in a recent poll.

"It's time for us to change our position," Wong told a press conference at which he announced he was disbanding the student activist group Scholarism, of which he is convenor, to form the new party.

He said the new party would aimed to nurture politicians who could push for "self-determination" among residents over the city's future.

Wong cannot himself stand for election until he is 21.

Hong Kong was returned to China by Britain in 1997 under a "one country, two systems" deal which for at least 50 years allows it much greater freedoms than seen on the mainland.

But there are fears that Beijing's influence is increasing and these freedoms are being eroded.

Some "localist" activists are calling for greater autonomy from Beijing or even independence -- an unthinkable option for Beijing.