I could find a report on Portuguese tenancy law by Bremen University. While legal aspects regarding private rental may be quite different from tenancy laws, I assume it still serves as an indication here. Citing (p. 138 f., footnotes removed):
Is the surveillance of certain parts (e.g. corridors) of the building lawful
The legislation regarding video surveillance for housing can be found in article 4, no. 4 of Law no. 67/98, 26th October, regarding the protection of personal data and, more specifically, in Law no. 34/2013, of 16th May, which regulates the use of systems of video surveillance by private security enterprises.
For the owner of a single unit house (i.e., not a block of apartments), the
installation of video surveillance equipment is subject to prior authorisation from the National Data Protection Commission (CNPD). The owner must fill out an application form and attach a layout with the positioning of the cameras and the locations covered by the angle of the capture of the images. Also, if there are cameras outside the house (for example, at the gates) there must be a warning or a sign to inform passers-by of data recording.
For a building or a condominium the applicant must also have the written
consent of all joint-owners or tenants (art. 6 Law 67/98) in order to protect the privacy of their personal and family life (articles 26 and 35 of the Constitution). The building must have a warning informing people that it is subject to video surveillance.
The landlord must inform the new tenants that the building is subject to video
surveillance and obtain their consent trough a clause in the tenancy agreement.
Such practice is not very usual. The reasons may be related to the high costs
of the equipment, the maintenance and the relative security and low criminality that the country enjoys. However, in more recent years we are witnessing a significant growth in the demand for these services.
Given that, the references to the constitution and protection of privacy and need for proper signage and consent, and reading between the lines of your question that you have not been informed about these cameras, my guess is that they are illegal the way they are currently used.
In your case I would consider the following steps:
- checking your rental agreement for hidden clauses agreeing to such practices
- covering the cameras by taping paper in their vision field (not damaging them)
- contacting the owner and
- asking about those cameras
- demanding them to be turned off
- asking whether video or audio has been recorded and demanding any such material to be deleted