More than 700'000 people or organizations were targeted, usually people on the left: unions, feminists, environmentalists etc.
A friend of my father runs an independent book store and he requested his files after the scandal was made public. He received a stack of paper over 10 inches high. The government pretty much had every part of his life on file. From mundane stuff to him participating in demonstrations (protesting for women's suffrage * , environmental issues). This is a guy who was never arrested in his life. Yet they had a record of pretty much everything.
This was in a time with limited technological capabilities.
In the 70's, the police probably took photographs of public gatherings and sent them to a special group which in turn had to identify the participants with the help of a magnifying glass and reference files.
Nowadays with have facial recognition techniques, cameras everywhere. Yes, this is some Public Enemy No. 1 shit, but it's a good time to be paranoid.
You can be sure that every form of electronic communication is in some form or another under surveillance by governments around the globe.
* Switzerland finally allowed women to vote in... drum roll... 1971.