The needle of suspicion in Friday's bomb blast in the Mecca Masjid points towards involvement of terrorist groups. A sophisticated mechanism was used to trigger the blast, police said.
Home Minister K Jana Reddy said the blast, which killed six people and injured 35, could be the act of foreign elements.
Three unexploded bombs recovered from the mosque had cell phones connected to them. "This is a sophisticated mechanism and we believe that this capacity is not available in Hyderabad," said Director General of Police MA Basit.
Police suspect that the bomb was kept in a tiffin box and triggered with the help of a cell phone. The exact nature of the explosives used could not be ascertained and the central government is rushing a team of its forensic experts.
"The unexploded bombs which were defused have been sent to the forensic laboratory and we will get some information in one or two days," said Hyderabad Police Commissioner Balwinder Singh.
Police and intelligence agencies had always believed Hyderabad to be a hub of agents of Pakistan's Inter Services Intelligence (ISI) and the terror groups supported by it. Several people were arrested on charges of links with these groups during the last 10 to 15 years.
Though these groups had reportedly targeted a temple, a cinema theatre and a police office in the past, this is the first time that a mosque has been targeted.
One policeman was killed in the suspected suicide blast at Hyderabad Police Commissioner's Task Force office in Begumpet in October 2005.
Police suspect that Friday's blast was aimed at disturbing peace and communal harmony in the old city, which has not witnessed any major communal riot in a decade.
Chief Minister YS Rajasekhara Reddy told newsmen in Delhi that some inputs were received from union home ministry and intelligence agencies during the last few months that some elements might try to disturb peace.
Communal riots in the city in 1990 had claimed more than 200 lives. The communally sensitive old city also witnessed riots in the late 1970s and early 1980s.
Muslims constitute 40 per cent of the city's four million population and are an overwhelming majority in the old city.