A weekend protest against online surveillance kicked-off on Friday at the headquarters of spy agency GCHQ in Cheltenham.
The protest had been organised by the We Are Anonymous Group, and is expected to be attended by hundreds of people supporting civil liberties groups questioning whether GCHQ's powers of internet surveillance and its assumed right to capture, store and share citizens' data are legal.
The complainants include Liberty, Privacy International and Amnesty International, as well as seven overseas human rights groups.
A tribunal had begun over a month ago, whereby lawyers representing the government and its associated spy agencies defended GCHQ's actions.
However, the government has not confirmed or denied whether it has monitored and used data in the way that has been spelt-out by documents leaked by former US National Security Agency (NSA) contractor Edward Snowden.
One of these documents revealed GCHQ's Tempora programme, which according to The Guardian was built up over five years by attaching "intercept probes" to transatlantic fibre-optic cables where they land on British shores.
Other documents indicate that data was routinely passed on from the UK to the NSA through the Prism surveillance programme, meaning that the private data of British citizens is being passed for use by a foreign power.
The Investigatory Powers Tribunal can investigate claims against MI5, MI6 and GCHQ. In this case it will seek to determine whether the Tempora and Prism programmes exist, and whether either violates articles 8 and 10 of the European Convention on Human Rights.
Lawyers representing the security services are likely to suggest that if mass surveillance had taken place, then it would have been in a way which adheres to the regulations on human rights.