A trove of newly published documents belonging to a purged Chinese leader has been launched at Hong Kong's book fair, despite fears Beijing is tightening freedom of expression in the city.

Former premier and Communist Party general secretary Zhao Ziyang was removed from China's top political leadership after he showed sympathy for students ahead of the bloody crackdown on their pro-democracy movement in Tiananmen Square in 1989.

He then spent 16 years under house arrest before his death in 2005.

The new book could anger Beijing and will strike a chord in Hong Kong, where some publishers have been scared off bringing out controversial titles after the disappearance of five booksellers from the city -- they later resurfaced in the mainland where one is still detained.

All worked for a company known for gossipy titles on Chinese leaders.

The material for the new book was brought out of China by Zhao's former aides, according to a publishing official who did not want to be named.

She said it would be "self-censorship" not to publish due to fears it could anger Beijing.

"These are not libellous writings...It is based on facts," she said.

The "Collected Works of Zhao Ziyang", published by Hong Kong's Chinese University Press, is a four-volume compilation of previously unseen policy documents, speeches and letters mainly by Zhao from 1980 to 1989, shortly before his fall.

While they do not mention the 1989 protests, they lay out his liberal views -- including pushing for democratic reform within the Communist Party and calling for less censorship of artistic works.

In one letter he reassures Hong Kong students during 1984 negotiations with its then colonial ruler Britain for the handover of the city back to China.

"You all can completely trust that the Chinese government will definitely take policies and measures, when resolving the issue over Hong Kong, in the wishes and interests of Hong Kong compatriots," he says.

Zhao is revered by Chinese human rights defenders, in part for opposing the use of force to quell the Tiananmen Square democracy protests, when hundreds of unarmed civilians -- by some estimates, more than 1,000 -- were killed.

The new book is for sale at the Chinese University Press booth at the annual book fair, with a constant flow of customers to the stall.

The fair has always been a source of books that would be banned on the mainland, with many readers crossing the border to browse titles banned at home.

Several publishers at this year's fair are still offering books likely to rile Beijing, despite the bookseller saga.

"This is a monumental effort in publishing," said Hong Kong-based publisher Bao Pu of the new book.

"Despite the pressure there are still efforts to do the right thing in the publishing industry," added Bao, who separately brought out Zhao's memoirs in 2009.

Hong Kong student leader pleads not guilty
Hong Kong (AFP) July 22, 2016 - A Hong Kong student protester pleaded not guilty Friday to multiple criminal charges over a demonstration against a pro-Beijing figure, a day after the conviction of an "Umbrella Revolution" leader was blasted by rights activists.

The court cases come with feelings running high in the city over fears that Beijing is tightening its grip.

Billy Fung, former president of Hong Kong University's student union, faces a raft of charges over a protest at the university in January where students stormed a council meeting.

Supporters gathered outside the court to support Fung Friday, a day after teenage protest leader Joshua Wong and two other prominent student activists were convicted for a protest that sparked major pro-democracy rallies in Hong Kong, known as the "Umbrella Revolution".

That prosecution was slammed by rights group Amnesty International, which said the "vague charges" against Wong, Alex Chow and Nathan Law "smacked of political payback".

Speaking outside the court Friday, Fung said he worried other students could be targeted, after pleading not guilty to four charges -- criminal intimidation, disorderly conduct in a public place, criminal damage and attempted forcible entry.

The charges all relate to the student siege of the meeting in January in protest at the appointment of pro-Beijing Arthur Li to a senior administrative role.

Around 200 students surrounded an HKU building and refused to let both Li, who is council chairman, and the vice-chancellor of the university leave the meeting, according to reports at the time. They said Li "forced" their hand due to his unwillingness to speak with them.

Fung, now 22, was accused of shouting: "Don't let him go! Don't let Arthur Li go! Kill him, kill him!" according to a court charge sheet.

He was released on a cash bail of HK$10,000 ($1,300) and will appear in court again in September for a pre-trial review.

The appointment of Li was made by Hong Kong's unpopular leader Leung Chun-ying -- chancellor of all the city's universities -- and comes after months of controversy and protests over what some see as politically motivated decisions at the prestigious university.

It also tapped in to wider concerns that Beijing interference is affecting academic freedom and freedom of expression in the city.

The former British colony was handed back to China in 1997 under a deal that guaranteed the retention of its civil liberties and capitalist system for 50 years.