Five people, including four policemen, were injured in an explosion inside the Pallabi Police Station in the city on Wednesday morning.   The injured policemen are -- Inspector (operation) Imran, Sub-Inspector Sajib Khan, two probationary sub-inspectors (PSI) Rumi and Ankush and Riaz.   The explosion took place around 7am when a weight-machine like object went off at the police station, leaving policemen injured, said DMP Deputy Commissioner (Media) Md Walid Hossain.   Rumi and Riaz were taken to Dhaka Medical College Hospital while Ankush was taken to the National Eye Science Institute & Hospital and two others, including Inspector Imran, were given first aid.   Additional Police Commissioner of DMP Krishnapodo Roy said police conducted a rain in Mirpur's Kalshi area around 2am after being tipped off that a group of was planning to carry out subversive activities. Three people -- Rafiqul Islam, Shahidul and Mosharraf -- were arrested and brought to Pallabi Police Station.   Two pistols and a weight-machine like object and some other equipment were recovered from the spot.