London Assembly member and Green Party peer Jenny Jones has discovered her actions have been recorded on a database of "domestic extremists" by the Met Police.
Documents obtained under a Freedom of Information request by the politician show that officers have been tracking her political movements since 2001.
Baroness Jones said the practice was "an absolute waste of time".
The Met Police claimed the force could not comment on individual cases.
Baroness Jones said: "When I got my file I was absolutely shocked to find it was three pages of essentially gossip and reporting on speeches I had made or tweets that I had made.
"At a time when they are finding it difficult to keep officers on the street, the fact they have got me on a database and they are keeping it up to date is ludicrous."
The documents list the peer's speaking engagements at events including police violence protests, a Stop the War rally in 2007 and a protest against the growing of genetically modified wheat crops in 2012.
They also record her social media activity, stating that she tweeted she was concerned about being "kettled" by the police during a vigil in 2012.
Baroness Jones, who used to be a member of the disbanded police watchdog the Metropolitan Police Authority said as a Green Party politician some of her views could be seen as extreme by some.
"They are using a label of 'domestic extremist' for people who are publicly elected, have never been arrested and have no criminal background. I do think it is a waste of their time," she said.
The Association of Chief Police Officers (Acpo) defines domestic extremism as individuals or campaign groups that carry out "criminal acts of direct action in furtherance of what is typically a single-issue campaign", adding that the groups usually seek to change legislation outside of the normal democratic process.
The Met said the National Domestic Extremism and Disorder Intelligence Unit database contains about 2,500 individual records.
The force added that the databases were "maintained in compliance with the Management of Police Information (MOPI) statutory code of practice".