Police used the national security law to stop local internet users from visiting the "Hong Kong Charter" website, following its failure to make an Israeli company remove the website.
The 2021 Hong Kong Charter website - which calls for the liberation of Hong Kong and an end to one-party rule - has been inaccessible since Friday.
The charter was initiated in March by eight exiled activists, including fugitives Nathan Law Kwun-chung, Sixtus Baggio Leung Chung-hang and Ted Hui Chi-fung.
Law said on Facebook that the website was banned by local internet service providers.
He appealed to Hongkongers to use VPN to avoid being "deprived by the Chinese Communist Party of the right to receive information."
A police official said the force would not comment on individual cases, stressing that any action would be in accordance with the law.
Under the security law, the police could require the person who published the offending information or the relevant service provider to either remove the information or provide assistance.
The police requested that the website's Israeli service provider Wix shut down the website this month. The page was pulled down but restored within a day. Wix said it took down the website by accident.