We present large scale 9' x 27' (25 pc x 70 pc) far-IR observations of the Sgr B2 complex using the spectrometers on board the Infrared Space Observatory (ISO). The far-IR spectra are dominated by the strong continuum emission of dust and by the fine structure lines of high excitation potential ions (NII, NIII and OIII) and those of neutral or weakly ionized atoms (OI and CII). The line emission has revealed a very extended component of ionized gas. The study of the NIII 57 microns/NII 122 microns and OIII 52/88 microns line intensity ratios show that the ionized gas has a density of n_e~10^{2-3} cm^-3 while the ionizing radiation can be characterized by a diluted but hard continuum, with effective temperatures of ~35000 K. Photoionization models show that the total number of Lyman photons needed to explain such an extended component is approximately equal to that of the HII regions in Sgr B2(N) and (M) condensations. We propose that the inhomogeneous and clumpy structure of the cloud allows the radiation to reach large distances through the envelope. Therefore, photodissociation regions (PDRs) can be numerous at the interface of the ionized and the neutral gas. The analysis of the OI (63 and 145 microns) and CII (158 microns) lines indicates an incident far-UV field (G_0, in units of the local interstellar radiation field) of 10^{3-4} and a H density of 10^{3-4} cm^{-3} in such PDRs. We conclude that extended photoionization and photodissociation are also taking place in Sgr B2 in addition to more established phenomena such as widespread low--velocity shocks.Peer reviewed