Hong Kong protests mark day city handed back to China

The anniversary of the handover on July 1, 1997, comes as tensions remain high after explosive revelations by a Hong Kong bookseller who said he was detained and interrogated for months in mainland China.

Lam Wing-kee, 61, was one of five city booksellers known for gossipy titles about mainland politicians who went missing last year and resurfaced over the border.

He will lead the protest march Friday.

Lam's story fanned growing concerns Beijing is tightening its grip on semi-autonomous Hong Kong, which is ruled under a "one country, two systems" deal enshrined in the handover agreement, guaranteeing its freedoms and way of life.

Various sectors of society traditionally air grievances at the annual July 1 march -- this year protesters will call for unpopular city leader Leung Chun-ying to resign after he recently hinted he may stand for a second term in 2017.

Leung and his government are seen by critics as stooges of Beijing and have been slammed for failing to help the booksellers.

Organisers say they expect 100,000 to turn out.

"They are angry with the current government and very disappointed at how things are managed," said organiser Jackie Hung of the Civil Human Rights Front.

Last year's 48,000 turnout was the lowest since 2008 as fatigue set in after mass pro-democracy rallies in 2014 failed to win political reform.

While the main march is expected to be peaceful, a separate pro-independence rally Friday evening outside China's liaison office could be a flashpoint.

Participants say they will wear black clothes and masks -- a poster advertising the rally showed mocked-up demonstrators wearing motorcycle helmets and carrying sticks.

Police have already warned they will take "resolute and effective actions against illegal acts" and say the pro-independence protesters have not asked for required permission for their rally.

Organisers say they want to hold the demonstration outside the China office out of anger at bookseller Lam's treatment.

"(Lam's case) is a very clear message to the world that China has already destroyed 'one country, two systems'," said Edward Leung, of Hong Kong Indigenous, which will take part in the evening protest.

Leung is part of the emerging "localist" movement which grew out of disappointment that the largely peaceful 2014 protests failed to win concessions.

Some localists say violence may be necessary to force change -- Leung himself is facing charges over clashes with police in the commercial neighbourhood of Mong Kok in February.

Localist activists have kept a low profile since then.

"They have been under some pressure (because) they haven't been able to do anything since the Mong Kok riots," said Ma Ngok, a political sciences professor at the Chinese University of Hong Kong.