In Andhra Pradesh, red alert has been sounded in Hyderabad following some unidentified persons opened fire at police in old city. <br/><br/>One constable belonged to Andhra Pradesh State Police was killed on the spot while two others injured in the incident. <br/><br/>As per the eye-witness, three persons wearing masks have come on a scooter and opened fire at the constables who were on a picket at Shalibanda area of old city.<br/><br/>City police Commissioner A K Khan said efforts are on to identify the culprits and full details would come out after an enquiry. <br/><br/>Thorough checkings are underway in the old city and other parts of the city for the past one month as a precaution as the May 18th coincides with the anniversary of bomb blasts at Macca Masjid few years back.