Those taken into custody also include 15 members of the banned Islamic outfit `Al-Umma', primarily from Coimbatore. Police and the State Government suspect this organisation to be behind the planting of explosives.

The largest number of preventive detentions, 130, were in Tiruchirapalli, followed by 177 in Sivaganga and over 50 in Coimbatore.

Meanwhile, investigations to trace the source of the explosive devices continued and security was beefed up. The ban on entry of visitors to Chennai airport is still in force.

A Coimbatore report said District Collector G Santhanam and Police Commissioner K Radhakrishnan reviewed the security arrangements in and around the Central Prison here.

Meanwhile, preliminary investigation, police said, led them to suspectthe involvement of seven fundamentalists in connection with the planting of the bombs. These seven, who are absconding, are also wanted in connection with the Coimbatore serial bomb blasts.

Police officials said they were pursuing some leads, but refused to say whether they were close to a breakthrough.

Meanwhile, two anonymous calls were received by the city police control room here saying the bombs had been planted in the police commissioner's office complex and that they were set to explode at 2 pm.

Police thoroughly checked the area, but nothing suspicious was found. The calls turned out to be a hoax. In a similar incident in North Chennai, an abandoned briefcase caused a minor flutter. However, an examination by the authorities revealed that it only contained some old clothes, police said.