The Democratic Party and the League of Social Democrats held separate protests outside Beijing's liaison office on Sunday over the disappearance of five men linked to a Causeway Bay shop that sells books which are critical of the central government.
Speculation has been growing that the five are being detained on the mainland.
Around 40 LSD protesters called on Beijing to clarify whether or not mainland law enforcement officers are secretly working on Hong Kong soil.
They also posted pictures of the missing men on the wall outside the liaison office in Western.
The Democratic Party's James To said the disappearances undermine Hong Kong's freedom of speech and publication.
Meanwhile, the Secretary of the Hong Kong Alliance in Support of Patriotic Democratic Movements of China, Lee Cheuk-yan, said the disappearances show how dangerous it would be to allow mainland immigration officers to be stationed in the SAR for the new cross-border rail line.
Lee said the arrangement could lead to Hong Kong people being arrested by mainland officials at the West Kowloon Terminus. He said he hoped even the pro-establishment camp would now reconsider the plan.
http://news.rthk.hk/rthk/en/component/k2/1233280-20160103.htm