The building was completely enveloped in flames when firefighters arrived, six bodies were found in the debris after the fire was extinguished.
Fire service spokesperson Paweł Frątczak said it was as yet unknown what caused the blaze, nor who the victims are.
Warsaw Police Headquarters’ Magdalena Bieniak said later that the burnt-down building was known to have been inhabited by homeless people.
Since Saturday, firefighters countrywide have been called in to 1,034 fires, in which 18 people died and 54 were injured. (PAP)
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