The first of three days of protests by online activists is due to take place at the UK Government Communications Headquarters.
Hundreds are expected to arrive for the demonstration in Cheltenham, which was organised by the We Are Anonymous group.
The group is supporting a legal challenge by civil liberty groups against the alleged use of mass surveillance programmes by UK intelligence services. The UK government has said interception is due to strict controls.
Among those challenging the legality of alleged "interception, collection and use communications" by agencies at the tribunal are Privacy International and Liberty.
The case centres on the alleged use by UK intelligence and security agencies of a mass surveillance operation called Tempora.
The Investigatory Powers Tribunal monitors whether the UK's spying laws are being observed and is seeking to determine whether the Tempora programme actually exists, and if so, whether it violates articles 8 and 10 of the European Convention on Human Rights.
The tribunal will also examine the UK's use of the US Prism mass data mining programme at a hearing in the Rolls Building in central London.
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