We present complete near-infrared (0.85-2.45 μm), low-resolution (~100) spectra of a sample of 26 disk L dwarfs with reliable optical spectral type classification. The observations have been obtained with the Near Infrared Camera and Spectrograph at the Telescopio Nazionale Galileo using a prism-based optical element (the Amici device) that provides a complete spectrum of the source on the detector. Our observations show that low-resolution near-infrared spectroscopy can be used to determine the spectral classification of L dwarfs in a fast but accurate way. We present a library of spectra that can be used as templates for spectral classification of faint dwarfs. We also discuss a set of near-infrared spectral indices well correlated with the optical spectral types that can be used to classify accurately L dwarfs earlier than L6.