The present disclosure provides various cyanobacterial extracts exhibiting antiviral activity to a wide spectrum of viruses, such as enterovirus (EV), respiratory syncytial virus (RSV), Human Herpesvirus (HHV), Ebola virus, porcine epidemic diarrhea virus (PEDV), and porcine reproductive and respiratory syndrome virus (PRRSV). The cyanobacterial extract is prepared from biomass of A. maxima (or Spirulina maxima). Also disclosed herein are process for preparing the cyanobacterial extract and uses of the cyanobacterial extract.