More than 40 pro-democracy organisations and individuals on Wednesday called on the government to apologise to Occupy leader Benny Tai, over its criticism of the academic's speech.
They also plan to hold a mass rally by Civic Square this Saturday to protest against attacks directed towards the Hong Kong university law lecturer.
The SAR and central government have condemned a comment Tai made last week in Taiwan about the hypothetical possibility of Hong Kong one day becoming independent, if China eventually becomes democratic.
In the statement, the pan-democratic groups said the legal scholar did not advocate the independence of Hong Kong, and was only talking about a hypothetical idea from an academic point of view.
It added that "ideas and discussions are not against the law", and everybody is entitled to freedom of expression under the Basic Law.
The pro-democracy figures said the authorities and pro-establishment groups are trying to create a chilling effect on freedom of speech in the city.
Hong Kong First lawmaker Claudia Mo, who is one of the signatories, said authorities are getting together for a calculated plot against Tai.
"The idea is to stifle free speech in Hong Kong and to send a chilling effect against the population that [says] 'Beware of what you say, or you will be taken to the court'," she said.
Mo told RTHK's Maggie Ho that Tai has become an "unwelcome person in the eyes of Beijing and the Hong Kong government" and warned that they would try to get rid of him from the University of Hong Kong.