According to the National Prison Guards’ Union (SNCGP) the action aims to defend their rights and interests and demand a review of their career status.
The guards also staged a vigil outside the Prime Minister’s official residence on Thursday as well as joining a civil servants’ strike on Friday, at 11am and 2pm.
There are currently around 4,000 guards in Portugal working in 50 prisons.
On Tuesday, day one of the strike action, the union estimated around 90 percent of the guards had stayed home.