It is OUR square and it is OUR right to protest in it whenever we want

"Sections 11-14 of the Public Order Act 1986 state that organisers of a 'procession' (ie a march/moving demonstration) must give prior NOTIFICATION to the police, and may have conditions imposed upon it, or be banned (ONLY with the assent of the Secretary of State, and only by the police applying to the COUNCIL of the district to issue such an order - the police themselves cannot do it). It does not, however, need prior PERMISSION. Organisers of an 'assembly' (ie a static demonstration) do NOT need to give notification, and though they may still have conditions imposed on them at the time of the assembly, these are more limited than those that can apply to a procession - furthermore, an assembly CANNOT be banned." - The above is paraphrased, with added emphasis.

Reference: Public Order Act 1986, which can be seen at http://www.statutelaw.gov.uk/

* I would recommend that you contact the police with a statement that reads as above, or similar, and take a copy or two of the relevant sections of the Public Order Act in their entirety. Then you can fight your case on a legal basis against the police on the scene if they turn repressive. Knowing your rights, and having proof of it, is a powerful weapon. I would also recommend you read the entirety of Part II of the Act, as there are particular sections on trespassory assemblies, delegation of powers, etc, which may be relevant to your case. *

NOTE: The legislation as it reads on that link has since been amended to change the meaning of "assembly" to include 2 or more people, rather than 20 or more. I am not aware of any other significant amendments.