The filament IRDC G035.39--00.33 in the W48 molecular complex is one of the darkest infrared clouds observed by textit{Spitzer}. It has been observed by the PACS (70 and 160,$micron$) and SPIRE (250, 350, and 500,$micron$) cameras of the textit{Herschel} Space Observatory as part of the W48 molecular cloud complex in the framework of the HOBYS key programme. The observations reveal a sample of 28 compact sources (deconvolved FWHM sizes $<$0.3 pc) complete down to $sim$$5 msun$ in G035.39--00.33 and its surroundings. Among them, 13 compact sources are massive dense cores with masses $>$$20 msun$. The cloud characteristics we derive from the analysis of their spectral energy distributions are masses of $20-50 msun$, sizes of 0.1--0.2 pc, and average densities of $2-20 times 10^{5} cmc$, which make these massive dense cores excellent candidates to form intermediate- to high-mass stars. Most of the massive dense cores are located inside the G035.39--00.33 ridge and host IR-quiet high-mass protostars. The large number of protostars found in this filament suggests that we are witnessing a mini-burst of star formation with an efficiency of $sim$15% and a rate density of $sim$$40 msun,$yr$^{-1},$kpc$^{-2}$ within $sim$8 pc$^2$, a large area covering the full ridge. Part of the extended SiO emission observed towards G035.39--00.33 is not associated with obvious protostars and may originate from low-velocity shocks within converging flows, as advocated by previous studies.
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