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We present the first paper of a series focused on the Blazhko effect in RRLyrae type stars pulsating in the fundamental mode, that are located in theGalactic bulge. A~comprehensive overview about the incidence rate andlight-curve characteristics of the Blazhko stars is given. We analysed 8\,282stars having the best quality data in the OGLE-IV survey, and found that atleast $40.3$\,\% of stars show modulation of their light curves. The number ofBlazhko stars we identified is 3\,341, which is the largest sample ever studiedimplying the most relevant statistical results currently available. Usingcombined data sets with OGLE-III observations, we found that 50\,\% of starsthat show unresolved close peaks to the main component in OGLE-IV are actuallyBlazhko stars with extremely long periods. Blazhko stars with modulation occurpreferentially among RR Lyrae stars with shorter pulsation periods in theGalactic bulge. Fourier amplitude and phase coefficients based on the meanlight curves appear to be substantially lower for Blazhko stars than for starswith unmodulated light curve in average. We derived new relations for thecompatibility parameter $D_{m}$ in $I$ passband and relations that allow fordifferentiating modulated and non-modulated stars easily on the basis of$R_{31}$, $\phi_{21}$ and $\phi_{31}$. Photometric metallicities, intrinsiccolours and absolute magnitudes computed using empirical relations are the samefor Blazhko and non-modulated stars in the Galactic bulge suggesting nocorrelation between the occurrence of the Blazhko effect and these parameters.

Blazhko effect in the Galactic bulge fundamental mode RR~Lyrae stars I: Incidence rate and differences between modulated and non-modulated stars