The Guardian, upon a freedom of information request.
Senior officers familiar with the program have indicated that many of the people listed on the database have no criminal record.
Police have allocated an especial unit called “the National Domestic Extremism Unit” to the cause using surveillance techniques to monitor campaigners who are listed on the secret database.
As Scotland Yard was battling to contain the fallout over the activities of a former undercover police officer who was asked to dig for "dirt" that would undermine the Stephen Lawrence campaign, evidence emerged that the main witness to his murder was also targeted.
Sources indicated that the Met secretly bugged meetings with Duwayne Brooks and his solicitor.
The surveillance operation was understood to have been authorised by a "senior officer" in around 1999 or 2000.
At least two meetings are believed to have been covertly recorded, one of them at the offices of Brooks's solicitor, Jane Deighton. She told the BBC that if true the operation was "scandalous".
The extremism unit monitors the full range of activists: from far-right activists in the English Defence League through to animal rights protesters, anti-capitalists and anti-war demonstrators.
British police put around 9 thousand political activists on its extremists list, many of whom have no prior criminal record, local media reported.
UK police documents show thousands of people across Britain are being monitored as “domestic extremists” -- Many of them with no criminal record.
According to the documents, police are using spying techniques including undercover agents, paid informants and intercepts to gather information on the listed individuals.
Details of the spying program were disclosed to