Comedian and activist Mark Thomas and five other people are launching legal action against the Met Police for monitoring their journalistic activity.
The group wants police to destroy files in a secret Scotland Yard database.
They allege that the Yard's National Domestic Extremism and Disorder Intelligence Unit is unlawfully holding information on them.
The Met said it would respond to the notice it had received of the action in due course.
The six, backed by the National Union of Journalists, are also seeking to bring legal action against the Home Office.
The group, including Mr Thomas, have all worked on media reports relating to alleged misconduct by corporations or public bodies and have previously brought complaints that the police have interfered with their rights as journalists.
They say they discovered their activities were being monitored after making requests to the Scotland Yard extremism unit about what information was being held on journalists.
One of the six found that his police file ran to 12 pages despite his having no criminal record. It included notes on his clothing and associates. Another complainant says he received a file which included large chunks of information blacked out.
The Metropolitan Police has confirmed in a Freedom of Information Request that it holds 2,000 records relating to journalists and photographers - but it will not say how many individuals that information covers.
Jules Mattsson, the Times reporter who uncovered that cache, is one of the six taking part in the legal action.
He said: "In the disclosed information from my file, there isn't even a hint that I'm suspected of any offence, nor do I have a criminal record.
"Instead the entries held about me contain such obvious statements as the fact I am 'always looking for a story' and 'has previously recorded police officers'."
Mark Thomas said: "The fact that none of the journalists are suspected of criminality but all of them cover stories of police and corporate wrongdoing hints at something more sinister, that the police seem to be spying on those who seek to hold them to account.
"The inclusion of journalists on the domestic extremist database seems to be a part of a disturbing police spying network, from the Stephen Lawrence family campaign to Hillsborough families, from undercover officers' relationships with women to the role of the police in the construction blacklist."
In their legal action, which is at its earliest stage before entering court, the six demand that the police destroy data held on them because the files breach their rights to privacy, freedom of expression and assembly.
A spokesman for the Metropolitan Police said: "We can confirm that we have received a letter before action which we will respond to in due course."
A spokesperson for the Home Office said it did not comment on potential legal proceedings.
The other journalists in the legal challenge are Jason Parkinson, Jess Hurd, David Hoffman and Adrian Arbib.