We study X-ray transients from the XMM-Newton slew catalog with focus on the identification of flare stars. We have selected 118 variable X-ray sources from the XMM-Newton slew catalog Delta-3 on the basis of at least 70-fold enhanced count rate with respect to ROSAT. Our characterization of the counterparts includes a systematic cross-match with optical and infrared photometric catalogs. The photometry was used to identify candidate stars for spectroscopic follow-up. Optical low-resolution spectra were obtained for 34 of those objects with OSIRIS@GranTeCan, most of them without any previous spectroscopic data. Among these objects we find many M-type stars, some giant stars and a small number of extragalactic sources. We present here the X-ray, optical and near-infrared properties of all slew transients from our sample together with the statistics of the various source populations.