A portable thermal printer, configured to be stand-alone or integrated in a handheld multifunction device, includes a printing mechanism having a paper pressing roller positioned in proximity to a paper output opening or slot with reversible constraint member, and a mechanism for the release of the paper pressing roller, the reversible constraint member being configured to engage a rotation pin of the paper pressing roller. The insertion/extraction of the rotation pin in/from the constraint member is performed by translating the paper pressing roller in an exit direction lying on an exit plane, and the release mechanism is configured to exert a force on the paper pressing roller in a direction parallel to the exit direction.