The alliance said in a statement that it was ordered by the force last Friday to remove all content on its online platforms within a week, citing national security rules.
From 10pm, it disabled its official website, and cleared out entries on its Facebook, Twitter, Instagram and YouTube accounts.
However, it said it doesn’t own or operate the June 4 online museum.
The group added that it has since started a new Facebook page to disseminate information.
The alliance is facing a charge of inciting subversion of state power, as are three of its leaders or former leaders – Lee Cheuk-yan, Albert Ho and Chow Hang-tung.