He said public libraries have to make sure materials available to citizens "serve the interest" of the city without breaching the national security law. This comes after the Leisure and Cultural Services Department - urged by government audit officials last month to remove books that posed national security risks - was found to have pulled books written by pro-democratic heavyweights, including the late Szeto Wah, and publications related to the 1989 Tiananmen Square crackdown.

Lee defended the decision to pull books from public libraries, insisting: "These books are accessible by people in private bookshops. If they want to buy, they can buy."

Asked whether the government had clear and transparent censorship criteria and if its lack of them would undermine freedom of speech and access to information in Hong Kong, Lee said "messages that are not in the interest of Hong Kong" that conveyed in the title, "may not be suitable for circulation" in public libraries.

"That is the important thing that I think any government needs to do, so that the books that we allow for public circulation do serve the interests of Hong Kong," he said.

"And I have strong confidence in the professionalism of the colleagues involved."

Lee added that the sentencing of a Hongkonger with US citizenship in the mainland on spying conviction shows that the Hong Kong government should be vigilant against lurking national security risks.

Among titles pulled from public libraries was Szeto's posthumous memoir The River of No Return, in which the late democrat details how his left-leaning ideals were dashed in his younger years when the Chinese Communist Party took power, and his efforts to fight for democracy in Hong Kong, and in China.

Szeto founded the now-defunct organization for Hong Kong's annual June 4 vigils, the Hong Kong Alliance in Support of Patriotic Democratic Movements of China.

Despite his pro-democracy and social justice activism, which also included setting up the Democratic Party and the now-defunct professional teachers' union, Szeto remained on amicable terms with Beijing and the local pro-establishment camp. The Beijing government in 1984 invited Szeto to witness the signing of the Sino-British Joint Declaration, and to sit on the Hong Kong Basic Law drafting committee the following year.

After his death in 2011, then-chief executive Donald Tsang Yam-kuen said Szeto "loved China and loved Hong Kong, and was committed to democratization."

Also among the items taken down from libraries were books by political cartoonist Wong Kei-kwan, alias Zunzi, whose comic column was recently suspended by Ming Pao.

Books by critics, including Chinese University associate professor Ma Ngok, political scholar Simon Shen Xu Hui, as well as cultural studies scholar Hui Po-keung and former lawmaker Margaret Ng Ngoi-yee - currently out on bail under foreign collusion charges, have also been pulled.

A search for the authors on the Hong Kong Public Libraries website shows: "No results were found for your search request."

Romance novels and reviews of Chinese literary classics have also been pulled, in addition to titles on social class in Hong Kong and international relations.